The Counting of the Omer
7. There are other communities that permit [weddings and haircuts (1)] until, and including, Rosh Chodesh Iyar, that is, a total of sixteen days. Thus, thirty-three days remain prohibited until the festival of Shavuos. (One may, however, cut one's hair on the day before the festival).
Even according to this opinion, on the day of Lag B'omer (33rd day of the Omer) there are no prohibitions (and when Lag B'omer falls on Sunday, hair may be cut on Friday, as stated above).
There are other communities which permit [weddings and haircuts] until the day before Rosh Chodesh Iyar, and the prohibitions begin on Rosh Chodesh itself. The first day of the three days of preparation [for Shavous] ("shloshes yemei hagbalah") is included in the prohibited period. We apply, however, the principle that a portion of the day is considered as the entire day, and weddings and haircuts are permitted [from daybreak of that day] (2). According to this custom as well, on the day of Lag B'omer there are no restrictions.
[Although there are communities that follow each of these different customs,] within each individual community, a single custom should be followed. It is improper that some people follow one custom and others another (3).
(1) Shaving is included in the prohibition against cutting hair.
(2) That is, keeping the mourning restrictions during the night is counted as if they were kept for the entire 24 hour period.
(3) Nowadays, however, in large Jewish communities, there has been a gathering of people from different communities of the world, and each group is required to continue to conduct itself according to its own customs. This is permissible, because it is considered like two courts of Jewish Law in one city, each of which is permitted to rule according to its own decisions.