The Obligation
The Gemara (Berachot 8a) teaches, "One should always finish the Parashi’ot with the community [by studying] Shnayim Mikrah VeEchad Targum (the Parashah twice and Targum Onkelos once)." The Aruch HaShulchan (O.C. 285:2) clarifies that this is a rabbinical obligation. However, he is convinced that Moshe Rabbeinu instituted this rule when he instituted communal Torah readings. Yalkut Yosef (O.C. 285:12) rules that women are not obligated to study Shnayim Mikrah, as it is a time-bound positive obligation.
Reason for the Obligation
In the introduction to the Sefer HaChinuch, the author beautifully explains a reason for this obligation:
Our sages established that we should read a portion of the Torah every week in the synagogue to inspire us to observe the Torah … The sages also obligated us to study the Torah portion that is read in the synagogue at home each week to further enhance our understanding of the Torah.
The aforementioned Gemara notes that all those who engage in Shnayim Mikrah "have their days and years lengthened." The Gemara might be suggesting that Shnayim Mikrah significantly enhances the quality of life. Surely, the joy on Simchat Torah of one who has fulfilled his Shnayim Mikrah obligation is exponentially greater than that of one who has not. Moreover, the Shabbat of those who observe this Halachah is immensely enhanced.
Interestingly, the Tur and Shulchan Aruch present this Halacha in the context of Hilchot Shabbat. Rav Yosef Dov Soloveitchik told me that the primary time for Shnayim Mikrah is Shabbat. This explains why the Shulchan Aruch (Yoreh Deah 400:1) permits a mourner in Shiva to learn Shnayim Mikrah on Shabbat even though he is forbidden to learn Torah on Shabbat.
I also heard from Rav Soloveitchik (in a public lecture delivered at Yeshiva University) that each Shabbat is characterized by its Parashah. For instance, the Shabbat on which we read Parashat Ki Teitzei is not simply Shabbat; it is Shabbat Parashat Ki Teitzei. I suggest that while the public reading of Ki Teitzei characterizes Shabbat as Shabbat Parashat Ki Teitzei on the communal level, individual Shnayim Mikrah study characterizes the Shabbat as Shabbat Parashat Ki Teitzei for the individual.
Of course, the primary way that Shnayim Mikrah enhances one's life is by promoting fluency in our most basic and holy text, the Torah. Accordingly, even women who are not technically obligated to study Shnayim Mikrah receive an abundant reward for doing so.
A Defense for Those Who Do Not Study Shnayim Mikrah
There is a "Limud Zechut" (limited Halachic justification) for men who do not learn Shnayim Mikrah. The Beit Yosef (O.C. 285 s.v. Aval Misham) cites the Ra’avan’s view that Shnayim Mikrah is only an obligation for an individual who has not heard the communal Keri’at HaTorah. According to the Ra’avan, Shnayim Mikrah is merely substituted for Keri’at HaTorah.
However, the Beit Yosef notes that almost all Rishonim reject the view of the Ra’avan. For example, he cites the Rambam (Hilchot Tefillah 13:25) who writes, "Although one hears the communal reading of the Torah, he must study the Parashah every week Shnayim Mikrah VeEchad Targum." In fact, the Vilna Gaon (Bi’ur HaGra O.C. 285:1) specifically notes that the Shulchan Aruch rejects the opinion of the Ra’avan.
Therefore, those who do not study Shnayim Mikrah are not "sinners," but it is proper to study Shnayim Mikrah in addition to hearing Keri’at HaTorah in shul. All agree, though, that one must study Shnayim Mikrah if he did not hear the communal Torah reading.
When Must We Complete Shnayim Mikrah Study?
The Gemara does not specifically state that one must complete Shnayim Mikrah by a specific time. Tosafot (Berachot 8a s.v. Yashlim), however, state that it is preferable to complete Shnayim Mikrah before eating on Shabbat. In fact, the Magen Avraham (O.C. 285:2) cites the Shelah HaKadosh, who writes that it is preferable to complete Shnayim Mikrah on Friday after Chatzot (midday). This preference stems from Kabbalistic concerns. Tosafot note, though, that it is acceptable to complete Shnayim Mikrah study even after the meal. However, Tosafot believe that Shnayim Mikrah must be completed before Shabbat ends. Indeed, the primary opinion presented by the Shulchan Aruch states that one must complete Shnayim Mikrah before Shabbat ends.
Nevertheless, the Shulchan Aruch cites two lenient opinions that appear in the Rishonim. One lenient view allows one to study Shnayim Mikrah until the Wednesday after Shabbat on which we read the Parashah. This view is based on the Gemara (Pesachim 106a) that permits one to recite Havdalah until Wednesday if he forgot to do so on Motza’ei Shabbat. A second, even more lenient view allows one until Simchat Torah to finish Shnayim Mikrah. The Aruch HaShulchan (285:10) writes that this is a viable opinion. The Mishnah Berurah (285:12) cautions that all agree that it is preferable to complete Shnayim Mikrah before Shabbat ends.
When May We Begin Study of Shnayim Mikrah?
Tosafot write that the earliest time to begin Shnayim Mikrah study of a particular Parashah is after the Mincha on Shabbat afternoon, when we start to read from that Parashah. This opinion is codified by the Shulchan Aruch (O.C. 285:3, as understood by the Mishnah Berurah 285:7).
Tosafot view Shnayim Mikrah as a weekly obligation (i.e., that we must study Shnayim Mikrah of a particular Parashah within the week in which we publicly read it). However, the lenient opinion that believes that one may complete Shnayim Mikrah until Simchat Torah regards Shnayim Mikrah as a yearly obligation (i.e., that one must complete Shnayim Mikrah every year). Arguably, just as the lenient view permits completing Shnayim Mikrah late, it also permits starting Shnayim Mikrah as early as Parashat BeReishit. Thus, if one finds difficulty in completing Shnayim Mikrah during the year but can do so during a vacation period, he should take the opportunity and complete Shnayim Mikrah for the entire year during the vacation period. Rav Mordechai Willig told me that he agrees with this analysis.
Rashi or Targum Onkelos?
The Rosh (Berachot 1:8) and the Tur (O.C. 285) assert that Rashi's commentary to Chumash constitutes a viable alternative to Targum Onkelos for the study of Shnayim Mikrah. The Beit Yosef (O.C. 285 s.v. VeIm Lamad), however, cites the Ri (Rashi's great-grandson) as disputing this assertion. He thus rules that a "God-fearing individual" should study both Targum Onkelos and Rashi. Similarly, in the Shulchan Aruch (O.C. 285:3), Rav Karo rules that Rashi’s commentary serves as a viable alternative to Onkelos, but a "God-fearing person" should study both Rashi and Onkelos.
It appears from the Shulchan Aruch that if one has sufficient time to study either Onkelos or Rashi (but not both), one can choose either and that there is no preference between the two. The Mishnah Berurah and the Aruch HaShulchan also do not seem to indicate a preference between Onkelos and Rashi, although the Chayei Adam, Kitzur Shulchan Aruch, and Yalkut Yosef prefer Onkelos over Rashi.
Alternative Translations
Tosafot (s.v. Shenayim) cite an opinion that asserts that any translation of the Chumash into the local vernacular constitutes a viable alternative to Onkelos. Tosafot reject this opinion, stating that Onkelos is special because Onkelos not only translates the Chumash but also explains many obscure words and passages. Both the Mishnah Berurah (285:5) and the Aruch Hashulchan (285:12) cite Tosafot's view as normative. However, the Mishnah Berurah writes that if one cannot comprehend Rashi, he may use a Yiddish (or any other language) translation based on Rashi and traditional sources rooted in the Talmudic tradition.
Conclusion: Shnayim Mikrah Is Within Everyone’s Grasp
The study of Shnayim Mikrah is within the grasp of all. Ideally, one should plan and habituate oneself to making Shnayim Mikrah a priority Shabbat activity. If it is not feasible, one should endeavor to fulfill this Mitzvah as best as he can. If one diligently seizes the available moments in transit, waiting in line, or waiting for appointments, he will find the necessary time to complete Shnayim Mikrah. For many, two round-trip flights to Israel from New York suffice to complete a basic level of Shnayim Mikrah VeEchad Targum.
The investment is relatively small, but the return is priceless—to master Chumash with Rashi, our most basic texts.