120:6 The students of Rabbi Akiva died on 33 of the days during the period of counting the Omer. Therefore, the practice on these days is to observe a partial form of mourning: we do not get married or have hair cuts. (Those whose circumstances allow them to get haircuts on chol hamoed – see 104:11 – may certainly have their hair cut during the sefirah period – Bi’ur Halacha 493:1 s.v. nohagim shelo.) There are different customs when it comes to counting these 33 days. Some places count them from the first day of the Omer and therefore forbid these activities until the thirty-third day of the Omer (Lag b’Omer). However, when Rosh Chodesh Iyar falls on Shabbos – making it a day with two forms of sanctity: that of Shabbos and that of Rosh Chodesh – then we allow marriages and haircuts on Friday. From Lag b’Omer and on, we permit these things because the students stopped dying on the thirty-third day of the Omer. We therefore increase our joy a little on that day and we do not recite tachanun (which is omitted on joyous occasions). Even though there were still some deaths on that day, we consider part of the day like the whole, so one should not have a haircut or get married until the daytime and not do so at night. However, if Lag b’Omer falls on Sunday, one may have his hair cut on Friday in honor of Shabbos.
120:7 Some communities permit haircuts and weddings through Rosh Chodesh Iyar, which is 16 days. This leaves 33 days until Shavuos during which time these things are prohibited; these communities permit haircuts on the day before yom tov. In any case, these things are permitted on Lag b’Omer itself. When Lag b’Omer falls on Sunday, we permit them on Friday, as mentioned above. Some communities permit these things up to but not on Rosh Chodesh Iyar, starting the prohibitions on the first day of Rosh Chodesh. The first of the three days of preparation for Shavuos is the thirty-third day of prohibitions but we say that part of the day is like the whole and we permit marriages and haircuts on it. These things are also permitted on Lag b’Omer, as discussed above. Each community must follow one practice and not mix and match pieces from different practices. (A person may follow the stringencies of more than one practice and it’s not considered self-contradictory because he is only doing so out of doubt, but it is not necessary to do so – Mishnah Brurah 493:17.)