The Torah tells us that Moshe was instructed to tell Aharon and his sons, who were the Kohanim, to bless Bnai Yisrael, and to provide them with the precise text of that blessing (במדבר ו':כ"ג-כ"ו). This blessing, of course, is known as the Birchas Kohanim, and because, as stated by the Mishnah in Sotah (דף ל"ח.), and explained by the Gemara there (שם), the Kohanim must raise their hands when reciting this blessing, a ruling codified by the Rambam (פרק י"ד מהל' נשיאת כפים הלכה ג') and by the Shulchan Aruch (אורח חיים סימן קכ"ח סעיף י"ב), the act of the offering of this blessing by the Kohanim to the people is often referred to as "נשיאת כפים," "the raising of the hands." The Gemara later in Sotah (עמוד ב' שם) indicates that every Kohein is required to go up to the platform, known as the "דוכן," (the Duchan, and hence the term "Duchaning," popularly used to describe the act of the Kohanim pronouncing their blessing to the Jewish people), and recite this Beracha, and a Kohein who does not do so is in violation of three requirements outlined by the Torah (שם פסוקים כ"ג,כ"ז). The Rambam (פרק ט"ו שם הלכה י"ב) explains that in truth, this Kohein has actually merely failed to fulfill the one Mitzvas Asei which requires him to bless the Jewish people, and there is no other real violation, but he is nevertheless viewed as if he has violated three different requirements, and the Shulchan Aruch (שם סעיף ב') states this as well, although he adds that this is only under certain circumstances. In any case, it is clear that there is a Mitzvah for the Kohanim to bless the people by reciting the Birchas Kohanim, as enumerated by the Rambam in his Sefer HaMitzvos (מצות עשה כ"ו), by Rabbeinu Saadyah Gaon in his Sefer HaMitzvos (מצות עשה קנ"ה), by the Semag (מצות עשה כ'), and by other Monei HaMitzvos who wrote Seforim which list and explain the Mitzvos of the Torah.
The Sefer HaChinuch (מצוה שע"ח), however, while also listing the recitation of Birchas Kohanim as a Mitzvah, adds that this Mitzvah must be performed daily, and the Rambam, in his introduction to the Hilchos Tefillah section of his Mishneh Torah, where his presentation of the laws of Birchas Kohanim are found, also writes (כותרת להל' תפילה) that there is a Mitzvah upon the Kohanim to bless the Jewish people every day. The Minchas Chinuch (שם אות א') notes that every time that a Kohein recites the Birchas Kohanim, even during the course of the same day, he fulfills a new Mitzvah, explaining that the basic Mitzvas Asei is to recite Birchas Kohanim at least once a day, so that once the Kohein has recited it one time during a particular day, he avoids missing the fulfillment of this once a day requirement, but if he is able to recite it more than once, he certainly is credited each time for the performance of a Mitzvah. The Sefer HaChinuch (שם) also states that this Mitzvah to bless the Jewish people daily is incumbent upon Kohanim in all places and at all times, meaning that it applies even when there is no Beis HaMikdash, which appears to also be the position of the Rambam (פרק י"ד שם), based on the aforementioned Mishnah in Sotah (דף ל"ז:-ל"ח.) which draws certain distinctions between Birchas Kohanim in the Beis HaMikdash and Birchas Kohanim elsewhere, but which nevertheless demonstrates that there is a Mitzvah to have Birchas Kohanim even outside the Beis HaMikdash. This view may also be based on the simple reading of the Tosefta in Sotah (פרק ז' הלכה ה'). The fact that Birchas Kohanim remains a Mitzvah today is also clear from the inclusion of its details and Halachos in the Tur (או"ח סימן קכ"ח-ק"ל) and the Shulchan Aruch (שם), because those Seforim present only those Halachos which are still observed today, even in the absence of the Beis HaMikdash.
It appears from the Magen Avraham (שם סימן קכ"ח ס"ק מ"א) that this requirement to recite Birchas Kohanim even today is MideOraisa, as he says is implied by Tosafos in Sotah (דף ל"ח: בד"ה כל), based on the Yerushalmi in Nazir (פרק ז' הלכה א', דף ל"ג:-ל"ד.), and this also seems to be the position of the Pri Megadim (באשל אברהם שם ס"ק מ"א ובמשבצות זהב שם ס"ק ג') and of Rav Yaakov Emden (שו"ת שאילת יעב"ץ חלק א' סימן נ"ד); the Keren Orah, in his commentary on the aforementioned Mishnah in Sotah (דף ל"ז:-ל"ח. שם בד"ה ברכת), discusses this issue at some length and brings several proofs, including a statement in the Yalkut Shimoni (חלק א', רמז תשי"א בד"ה ושמו), to support his conclusion that even outside of Eretz Yisrael, the Mitzvah of Birchas Kohanim is MideOraisa. The Sdei Chemed (דברי חכמים סימן ס"ד, ועיין עוד שם בכללים, מערכת הנו"ן, כלל ל"ט) also discusses this question, however, and notes that there may be some authorities who hold that the Mitzvah of Birchas Kohanim today is only MideRabbanan, including Rav Yaakov Emden, in an apparently contradictory statement elsewhere in his writings (ספר מור וקציעה לשו"ע או"ח שם), while Rav Yerucham Perlow, in his commentary on the Sefer HaMitzvos of Rabbeinu Saadyah Gaon cited above (שם), suggests that this may be the opinion of Rabbeinu Saadyah Gaon as well, and he refutes some of the proofs that others offer to demonstrate that Birchas Kohanim is MideOraisa even outside of the Beis HaMikdash. The Sdei Chemed (שם ושם) concludes, though, that the clear majority of authorities hold that Birchas Kohanim is MideOraisa even outside of the Beis HaMikdash, and even nowadays; it is noteworthy that Rav Yerucham Perlow (שם) quotes some who hold that the Behag, because he presents the Mitzvah of Birchas Kohanim twice in his list of the Mitzvos (מצות קום עשה קצ"ח, פרשיות המסורים לצבור ל"ב), actually believes that there is a separate, independent Mitzvah MideOraisa to recite Birchas Kohanim outside of the Beis HaMikdash, although he himself thinks that this understanding is incorrect. The Aruch HaShulchan (או"ח שם סעיף ב') writes clearly that Birchas Kohanim even today is MideOraisa, as do the Chayei Adam (כלל ל"ב סעיף כ"ח), the Ben Ish Chai (שנה ראשונה, פ' תצוה אות כ'),and others cited by the Kaf HaChaim (שם אות קס"ה), and the Mishnah Berurah (שם ס"ק ח') appears to concur, elaborating on his position elsewhere (בהערה לסעיף מ"ד שם, נדפס לאחר אות קל"ז בשער הציון) with several proofs.
The Mishnah in Taanis (דף כ"ו.) indicates that Birchas Kohanim is recited as part of as many as four Tefillos on certain days, namely, Shacharis, Mussaf, Minchah, and Neilah; the Gemara there (עמוד ב' שם) elaborates on this, but concludes that Birchas Kohanim is not recited at Minchah because, as explained by Rashi (שם בד"ה כל יומא), on an ordinary day, the Kohein will already have eaten and drunk by Minchah time and it is possible that he will have had too much to drink and thereby be labeled a שכור, one who is drunk, and as the Gemara states further on (שם), a שכור can not recite Birchas Kohanim, a ruling codified by the Rambam (פרק ט"ו שם הלכה א',ד') and by the Shulchan Aruch (שם הלכה ל"ח). Even on a fast day, therefore, when it may be assumed that the Kohein did not drink anything, Birchas Kohanim should not be recited at Minchah for the sake of being consistent, although the Gemara (שם) states that where the practice is to recite Minchah on a fast day very late in the afternoon, so that it is in some ways similar to Neilah, Birchas Kohanim can indeed be recited then (ועיין שם ברש"י ד"ה כיון ובד"ה כתפלת ובתוד"ה והאידנא); the Rambam (פרק י"ד שם הלכה א',ב') and the Shulchan Aruch (שם סימן קכ"ט סעיף א') rule accordingly. It is clear, though, that the Kohanim should recite Birchas Kohanim at least once a day, as part of Tefillas Shacharis, if not more than that.
The above cited Gemara in Sotah (דף ל"ח:) indicates that the Kohanim are to go up to the platform in order to recite Birchas Kohanim during the Beracha of the Shemoneh Esrei that is known as "Avodah," "service of Hashem," which begins with the word "רצה," because an earlier Posuk in the Torah (ויקרא ט':כ"ב) states that Aharon HaKohein blessed Bnai Yisrael as part of his Avodah, as explained by Rashi in Sotah (בד"ה וירד שם), and therefore Kohanim today should do the same; the Rambam (שם הלכה ג') and the Shulchan Aruch (שם סימן קכ"ח סעיף ח') both rule that the Kohanim ascend the platform when "רצה" is being recited. The Aruch HaShulchan (שם סעיף א') notes that in the Beis HaMikdash, Birchas Kohanim would be recited after the offering of the Ketores and the limbs of the Korban Tamid, as indicated by the Mishnah in Tamid (פרק ז' משנה ב') and codified by the Rambam (פרק ו' מהל' תמידין ומוספין הלכה ה'), and therefore nowadays as well, when our Tefillos take the place of the Korbanos, Birchas Kohanim should be recited after the main body of the Tefillah (meaning the Shemoneh Esrei) has been completed, which means after the Beracha of "שומע תפלה" that concludes the "request" section of the Shemoneh Esrei is done, and the Beracha starting with "רצה" begins. This is the point, then, when the Kohanim ascend the platform to recite the Birchas Kohanim. The Gemara in Megillah (דף י"ח.) states, though, that the actual blessing of Birchas Kohanim is not recited until after the Beracha of "הודאה," meaning "thanksgiving" is completed, prior to the Beracha which begins with the words "שים שלום," based on Pesukim and reasons presented there; the Rambam (הל' נשיאת כפים שם) and the Shulchan Aruch (שם סעיף י') rule accordingly, and the Aruch HaShulchan (שם סעיף ב') elaborates upon this procedure, explaining that Birchas Kohanim ends with the Beracha of peace (עיין במדבר שם פסוק כ"ו) and it is thus juxtaposed to the final Beracha of the Shemoneh Esrei which also focuses on peace.
It seems clear from all of the above that Birchas Kohanim is intended to be a part of the daily davening, and this indeed seems to be what is required by the Rambam (שם הלכה א'-ג') and by the Shulchan Aruch (שם סימן קכ"ט סעיף א'), as explained by the Mishnah Berurah (שם ס"ק א'); it is in fact the practice of Sephardic Jews to have Kohanim recite the Birchas Kohanim on a daily basis at the appropriate time, and according to the Mishnah Berurah (שם סימן קכ"ח ס"ק קס"ד), this is the accepted practice in Eretz Yisrael and its vicinity as well. The Ramo, however (שם סעיף מ"ד), writes that the custom in all Ashkenazic communities is that the Kohanim do not recite the Birchas Kohanim except on Yomim Tovim, and he explains that this is because only on Yom Tov is one truly in a state of Simcha, happiness, because of the notion of Simchas Yom Tov, which he indicates is a prerequisite for the recitation of Birchas Kohanim; on other days, though, including Shabbos, people are nervous and worried about making a living and earning their livelihood, and the Kohanim can therefore not recite Birchas Kohanim. The Ramo elaborates on this explanation somewhat in his Darkei Moshe on the Tur (שם אות כ"א). He also notes in the Shulchan Aruch (שם) that even on Yomim Tovim, Birchas Kohanim is recited only at Mussaf, because this is when the level of Simcha is at its highest, although he points out that some communities recite it at Shacharis of Yom Tov as well, and he also states that Birchas Kohanim is recited on Yom Kippur; the Mishnah Berurah (שם ס"ק קס"ו) explains that this is because there is a feeling of Simcha on Yom Kippur due to the attainment of forgiveness for one's sins.
Other reasons are suggested for this Ashkenazic custom to have the Kohein recite Birchas Kohanim only on Yomim Tovim and on the Yomim Noraim; the Maharil (שו"ת מהרי"ל החדשות סימן כ"א), for example, writes that Kohanim had the practice at one time to go to the Mikveh prior to reciting Birchas Kohanim, and since it was difficult, especially in the winter time, in the colder climates, to go to the Mikveh every day, the practice developed to recite Birchas Kohanim only on Yomim Tovim (and on the Yomim Noraim) because people then would go to the Mikveh anyway. The Maharil also indicates (שם) that reciting the Birchas Kohanim daily causes an imposition on the Tzibbur, because people are in a hurry to go to work or to learn, and thus resent the delay, an explanation also suggested by the Kol Bo (סימן קכ"ה), and the Maharil (שם) also writes that for certain reasons, Birchas Kohanim should not be recited when non-Jews are in the Shul and because there were, apparently, non-Jews around the Shuls frequently, Birchas Kohanim was limited to only certain times of the year. The Beis Ephraim (שו"ת בית אפרים סימן ו') offers yet another suggestion, noting that today we can not really be sure who is truly a Kohein and who is not, because we no longer have accurate records of family lineage, which are required in order to determine who is really a Kohein, as discussed by the Gemara in Kesubos (דף כ"ד:-כ"ה:), and since the Gemara there (דף כ"ד:) indicates that it is forbidden for a non-Kohein to raise his hands and recite Birchas Kohanim, a ruling cited in the Shulchan Aruch by the Ramo (שם סעיף א'), the custom developed to limit the occasions on which Birchas Kohanim is recited by (people who are assumed to be) Kohanim, while not doing away with Birchas Kohanim completely. Another explanation for this Minhag is suggested by the Chasam Sofer (שו"ת חתם סופר חלק או"ח סוף סימן כ"ג) who points out that Birchas Kohanim is connected to the Avodah in the Beis HaMikdash, as mentioned above, and consequently, just as the Avodah is invalid if not done properly and with the right intentions, so too Birchas Kohanim may not have its intended effect if the Kohanim who recite it do not do it properly and with the right intentions. Since people on a daily basis are occupied with thoughts of their work, it is difficult to have the correct Kavanah during davening and Birchas Kohanim; as a result, the custom is to have Birchas Kohanim only on Yom Tov when people are free of worries and can have the proper Kavanah.
Despite these reasons, though, many authorities consider it improper to have Kohanim recite the Birchas Kohanim only on Yomim Tovim and not on a daily basis. The Beis Yosef, in his commentary on the Tur (או"ח שם בסוף הסימן, בד"ה כתב האגור), cites one of the explanations presented above from the Maharil (שם), but he rejects it completely, saying that it is a weak defense of this custom, and he therefore praises those whose practice it is to have the Kohanim recite the Birchas Kohanim every day. Similarly, the Rama of Fano (שו"ת הרמ"ע מפאנו סימן צ"ה) writes that the custom for Kohanim not to recite the Birchas Kohanim every day of the year is a bad custom which should be corrected, and the Kaf HaChaim (שם אות רע"א) writes that it is proper not to try and find ways to avoid having Kohanim recite Birchas Kohanim daily, and thereby avoid fulfilling a Mitzvas Asei from the Torah, because it is always better to try and bring more Beracha into the world. It is reported in the Sefer Aliyos Eliyahu (דף כ"ב: בהערה א') in the name of Rav Chaim Volozhiner that the Vilna Gaon tried several times to institute the practice of having the Kohanim recite Birchas Kohanim daily in his Beis HaMidrash, but he was unsuccessful, and finally stopped pursuing the matter when he once got arrested on false charges and viewed this as a sign from Hashem that he should stop trying to change this custom. This story, as well as a similar one about Rav Chaim Volozhiner himself, who likewise tried to change this custom until a fire broke out and destroyed half the city, is also recorded by the Netziv (שו"ת משיב דבר חלק ב' סימן ק"ד).
Nevertheless, the Shaarei Teshuvah (שם ס"ק ע"ח) writes that this custom not to have Kohanim recite the Birchas Kohanim on a daily basis is a very old one among the Ashkenazim, and the Aruch HaShulchan (שם סעיף ס"ד), while admitting that there is no clear reason for this custom, cites those authorities who tried unsuccessfully to change it and asserts that it is as if a sign has come from Hashem that we are not to change our practice; he then suggests that it may be because Birchas Kohanim requires the intensity of Simcha possessed by Aharon and the Kohanim when the Mishkan was dedicated (עיין ויקרא פרק ט'), and this obviously is not attainable every day. The Aruch HaShulchan (שם) also writes that some have the practice not to have the Kohanim recite Birchas Kohanim even on Yom Tov, if it should happen to coincide with Shabbos, but he writes that this custom has no merit and should be abolished. The Taz (שם ס"ק ל"ח) also quotes and rejects this custom, although the Magen Avraham (שם ס"ק ע'), who likewise seems uncomfortable with the custom, does quote a reason for it, as does the Eishel Avraham (ספר אשל אברהם מבוטשאטש שם בד"ה עמ"א סק"ע) who says, among other things, that there was concern that people might carry water to Shul (without an Eiruv) for the Kohanim to wash with, and the recitation of Birchas Kohanim on Shabbos was thus eliminated. Most Poskim, however, including the Chayei Adam (שם סעיף י"א) and the Mishnah Berurah (שם ס"ק קס"ה), as well as Rav Moshe Feinstein (שו"ת אגרות משה או"ח חלק ג' סימן י"ח), assert that Birchas Kohanim should be recited on Yom Tov even if it coincides with Shabbos; the Mishnah Berurah elsewhere (שם סימן ק"ל ס"ק ד'), however, does state that on Shabbos, the special requests often recited by the Tzibbur while the Kohanim sing during Birchas Kohanim are omitted on Shabbos because such Tefillos are usually inappropriate on Shabbos. It should be noted that in most places in Eretz Yisrael, the Minhag even among Ashkenazim is to have Kohanim recite Birchas Kohanim daily, as reported by Rav Yechiel Michel Tukatchinsky (ספר עיר הקדש והמקדש, חלק ג' פרק כ"ה סעיף ב)