Lag Ba’Omer marks the start of the third trimester of Sefirat Ha’Omer, the seven-week period beginning with Pesach and culminating on Shavu’ot. The two festivals which bookend this forty-nine day count correspond to the barley harvest and wheat harvest in the Land of Israel; consequently, the Torah prescribes an offering of wheat on Shavu’ot, known as the Shtei HaLechem, and an offering of barley on the second day of Pesach, known as the Korban Ha’Omer. This second Korban is of particular significance, as the Torah states, “VeLechem VeKali VeCharmel Lo Tochelu Ad Etzem HaYom HaZeh Ad Havi’achem Et Korban Elokeichem Chukat Olam LeDoroteichem BeChol Moshevoteichem,” “And bread, and toasted grain, and fresh grain, you shall not eat until the essence of this day, until you bring the offering of your God; [this shall be] an eternal law for all your generations, in all your habitations” (VaYikra 23:14). Until the Korban Ha’Omer is brought, the Issur Chadash (prohibition to eat new grain) is in force. But the destruction of the Beit HaMikdash necessitated a change in the timetable of Achilat Chadash; the Korban could no longer serve as a demarcation line. Faced with this dilemma, Rabban Yochanan Ben Zakkai declared, “SheYehei Yom HaNeif Kulo Assur,” “That the day of waving [the Omer; viz. the 16th of Nissan] should be entirely forbidden [with regard to Achilat Chadash]” (Sukkah 41a).
There are two rationales provided by the Gemara in explanation of Rabban Yochanan Ben Zakkai’s Takkanah. The first, anonymous elucidation is that this enactment is not De’Oraita in nature; on the Torah level, when is permitted to eat Chadash at sunrise of the 16th of Nissan (in the absence of a Beit HaMikdash or Mishkan). The Takkanah is due to our halachic concern that the Beit HaMikdash might be rebuilt on the night of the 16th, which would require one to wait until the Korban Omer was brought, but provide insufficient preparation time for the Chachamim to bring the Omer in a timely fashion. While the Beit HaMikdash was still standing, residents of far-off locales could be confident that the Omer would have been brought by Chatzot HaYom; but such a confidence would be unwarranted in the extraordinary scenario of Binyan Beit HaMikdash HaShlishi on the previous night. As such, Rabban Yochanan Ben Zakkai decreed that one should refrain from Chadash until nightfall, just in case the Chachamim needed more time.
Rav Nachman Bar Yitzchak presents a different rationale for this Takkanah. He argues that Rabban Yochanan Ben Zakkai concurs with Rabi Yehudah that “Min HaTorah Hu Assur DiChtiv Ad Etzem HaYom HaZeh VeKaSavar Ad Ve’Ad BiChlal,” “It is forbidden by the Torah, as it is written, ‘Until the essence of this day’, and [Rabi Yehudah] holds that ‘until’ is included (i.e. ‘until the 16th’ includes the 16th).” Accordingly, Rabi Yehudah and Rabban Yochanan Ben Zakkai are actually in agreement; Rabban Yochanan Ben Zakkai did not cut his Takkanah out of whole cloth, but rather, “Darash VeTikein,” “He expounded [the Pasuk] and [on that basis] decreed.”
Other instances of the phrase Be’Etzem HaYom point to a third interpretation. In the context of Yetzi’at Mitzrayim, both Targum Onkelos and Targum Yerushalmi (on Shemot 12:51) translate “Be’Etzem” as “BiChran,” “In the fullness,” and this rendering is also utilized by the Targumim elsewhere. Indeed, the Sifrei (Sifrei Devarim 337) states that Noach’s entry to the Teivah, Avraham’s Berit Milah, and Moshe’s death-- all events which took place Be’Etzem HaYom-- occurred “BaChatzi HaYom,” “At midday.” Why do none of the opinions in the Gemara regard Chatzot as the true meaning of Be’Etzem HaYom?
Perhaps the reticence of the Tanna’im and Amora’im to identify Chatzot as the proper time to eat Chadash, MiDe’Oraita, in the absence of the Beit HaMikdash, is due to the Torah’s emphasis on “Yom Hanifchem Et Ha’Omer,” “The day upon which you bring the Omer” (VaYikra 23:12). This may imply that there is a specific importance to the Yom itself, and therefore it should not be split up.
Alternatively, other factors may be boosting Heineitz HaChamah and Sheki’a, rather than inhibiting Chatzot, as a candidate for the Chadash benchmark. For sunrise, it is possible to argue that since the Omer may be brought from daybreak and onward, the temporal moment mentioned in the phrase, “Ad Etzem HaYom HaZeh Ad Havi’achem Et Korban Elokeichem,” “Until Etzem HaYom of this day, until you bring the offering of your God” (VaYikra 23:14), is sunrise (when it is not superseded by the Korban Omer). Conversely, one arguing in favor of sunset as the proper benchmark might say that since the Korban Omer may be brought all day (although, in practice, the Beit Din would generally bring it in the morning; Mishnah Menachot 10:5), the Pasuk is setting up sunset as the final time of the Issur Chadash. Additionally, most foods which become permitted at a set time (e.g. Terumah for an impure Kohein, Chameitz after Pesach) do so at sunset; accordingly, the ambiguity of Etzem HaYom should perhaps bend to this general trend.
The case for Sheki’a seems the strongest, and indeed Rambam (Hilchot Ma’achalot Assurot 10:2) rules in accordance with Rav Nachman’s interpretation of Rabban Yochanan Ben Zakkai and Rabi Yehudah that Chadash is prohibited MiDe’Oraita until sunset at the end of the 16th of Nissan. Etzem HaYom, thus, is taken to mean the completion of the day (Onkelos’s rendering of “BiChran Yoma” may also be interpreted in this manner). As such, Issur Chadash applies MiDe’Oraita until sundown at the start of the 17th of Nissan, until the restoration of the Beit HaMikdash, BiMeheirah BeYameinu.