- Peninei Halakha
There are many customs regarding when the mourning period begins and ends. We will mention the four primary ones:
1. The customs of mourning apply for the entire omer period. This custom is based on the version of the Gemara that appears in our texts (Yevamot 62b), which states that R. Akiva’s students died between Pesaĥ and Shavu’ot. Thus, one should follow the customs of mourning throughout that period.
2. The mourning period continues until Lag Ba-omer (the 33rd day of the omer). This custom is based on the well-known tradition that R. Akiva’s students stopped dying on Lag Ba-omer.
3. The customs of mourning apply until the 34th day of the omer. This is based on a Sephardic tradition, according to which the text of the Gemara reads: “R. Akiva’s students died until “pros ha-Atzeret.” Since pros means “half” (and Atzeret refers to Shavu’ot), this means that they died until half a month before Shavu’ot. When we subtract fifteen days from the 49 days of the omer, we are left with 34 days during which R. Akiva’s students died, during which we observe customs of mourning.
4. We observe 33 continuous days of mourning. This custom is based on a tradition that R. Akiva’s students died on every non-festive day of the omer period, which adds up to 33 days. Consequently, we must observe customs of mourning for 33 straight days, whether this period coincides with the beginning or the end of the omer period.
 Sources for the various traditions: 1. R. Natronai Gaon, R. Hai Gaon, and Ritz Gi’at write that ever since R. Akiva’s students died, we refrain from marrying between Pesaĥ and Shavu’ot. Tur §493 cites this opinion anonymously, indicating that it is the accepted opinion. Orĥot Ĥayim states this with regard to both weddings and haircuts (Minhagei Yisrael, vol. 1, pp. 101-102). Shibolei Ha-leket and Rabbeinu Yeruĥam provide two additional reasons why we mourn during the omer period: a) it is based on R. Yoĥanan b. Nuri’s opinion that the wicked are judged in Gehinom between Pesaĥ and Shavu’ot; b) these are days of judgment that determine the success of the grain harvest.
2 and 3. These customs are based on a tradition that R. Akiva’s students stopped dying on Lag Ba-omer. Several Rishonim mention this, including Me’iri (Yevamot 62b): “The Ge’onim had a tradition that the dying ceased on the 33rd day of the omer. Therefore, we have a custom to refrain from fasting on that day. This is also why there is a custom not to get married between Pesaĥ and that day.” There is also a different version of the Gemara in Yevamot, as Sefer Ha-manhig states in the name of Ha-ma’or, that according to a Sephardic variant of the text, R. Akiva’s students died until pros ha-Atzeret. Since pros implies half a month, or fifteen days, it turns out that we must observe customs of mourning until the 34th day of the omer. There is a difficulty with this approach: According to the tradition mentioned above, the students stopped dying on the 33rd, but according to the calculation of pros, we mourn until the 34th. There are two opinions regarding this issue. Some maintain that we must observe the customs of mourning until the 34th day. This is the view of Ibn Shu’ib and Tashbetz 1:178, cited in Beit Yosef §493. sa 493:2 concurs. Perhaps these authorities explain that the students continued dying throughout the day of the 33rd; therefore, the mourning period ends only on the 34th. In contrast, Ha-manhig (Hilkhot Erusin, end of §106) implies that the period of mourning actually ends on Lag Ba-omer. Other Rishonim and Aĥaronim write similarly. According to them, we must explain that when the Sages use the word pros, they mean approximately half a month, because in reality we stop mourning sixteen days before Shavu’ot. So writes R. Yaakov b. R. Avraham Castro (Maharikash) in Erekh Leĥem (see also below n. 3).
4. The Rishonim cite a tradition in the name of Tosafot (not printed in our text of the Gemara) that R. Akiva’s students died on the 33 ordinary, non-festive days of the omer period. If we subtract from the 49 days of the omer six days of Pesaĥ, isru ĥag (the day after Pesaĥ, six Shabbatot, and three days of Rosh Ĥodesh, we are left with 33 days on which the students died. Consequently, we observe customs of mourning for 33 consecutive days. Some observe them at the beginning of the omer, while others do so at the end. Beit Yosef cites this tradition in the name of Ibn Shu’ib, and Rema 493:3 mentions it as well. Baĥ, mb 493:13, and bhl ad loc. clarify the details of this practice. Many Ashkenazim observe the customs of mourning during the latter part of the omer, because the Crusades – during which their ancestors experienced terrible massacres – began in the months of Iyar and Sivan. On the eighth of Iyar, the Jews of Speyer were massacred; the Jews of Worms on the 23rd of Iyar; the Jews of Mainz on the third of Sivan; and the Jews of Cologne on the sixth of Sivan. The earliest custom in this regard was to begin the mourning period on the second of Iyar and end it on Erev Shavu’ot. Nonetheless, one who follows this custom may start the mourning period at the beginning of the omer as well. Even though the laws of mourning do not find expression on Ĥol Ha-mo’ed Pesaĥ – as there is a mitzva to rejoice throughout the holiday – this does not take away from the 33 days, just as the days of Shabbat, on which mourning is precluded, count toward the 33 days of the omer and the seven days of regular mourning.
Minhagei Yisrael, vol. 1, pp. 101-111, explains the Sephardic and Ashkenazic customs. In his addenda, vol. 4, pp. 237-241, he brings proofs that the word pros does not usually mean half, but “close to.” According to this, the version that reads “until pros ha-Atzeret” means until Erev Shavu’ot. It is important to note that there was another custom: to observe mourning throughout the omer period, except on Rosh Ĥodesh and Lag Ba-omer, when everything is permitted. We do not follow this practice (cited in ma 493:5 and mb 493:15).