The Tur writes that during the days of Sefira the students of Rebbe Akiva died, and therefore it is customary to not hold weddings during that period.

We know the aveilus is to be for a period of thirty three days, and there are divergent opinions in how to count them:

1. The Shulchan Aruch (493:2) writes that we begin from the second day of Pesach through Lag B’Omer, after which all mourning ceases; haircuts are allowed the morning of the 34^th
2. Begin on the second day of Pesach, but the mourning lifts on the 33^rd day, not the 34^th (Rema commenting on Shulchan Aruch’s opinion)
3. Begin the day after Rosh Chodesh Iyar, and continue until Erev Shavuos with the exception of Lag B’Omer. (Rema in 493:3 and Magen Avraham)
4. Begin on the First day of Rosh Chodesh Iyar until the morning of the Sheloshos Yemei Hagbala
5. Begin after Isru CHag Pesach, until Rosh Chodesh Sivan, with breaks for Rosh Chodesh Iyar and Lag b’Omer (MB 493:15)
6. Begin Pesach and go all the way through Shavous with breaks for Rosh Chodesh only (Magen Avraham ibid)

Rav Moshe (OC1:159) explains, based on the Vilna Gaon, that there are two fundamental ways to understand the aveilus:

A. We can be sad on the days that the students of Rebbe Akiva actually died. This is the reasoning of the first two customs (1) and (2), they stopped dying 15 days before Shavuos (the 34^th of the Omer), or on the 33^rd of the Omer (Biur Hagra). This is also the reasoning of (6), they died throughout sefira, with the exception of the joyous days – 7 days of Pesach, 6 Shabbosim, and 3 days of Rosh Chodesh.  (49-16=33). So we group together reasons (1), (2) and (6) as commemorating the actual days of death.

B. We know they died for 33 days, and we are not particular to mourn on those specific days, but choose 33 days (including the Shabbosim) to mourn in commemoration. This is the thinking of reasons (3), (4) and (5):

(3) Says we start after Rosh Chodesh Iyar, leaving 28 days in Iyar and 5 days in Sivan
(4) Says we start on the first day of Rosh Chodesh Iyar, leaving 30 days in Iyar and 3 days in Sivan. According to both of those Lag B’Omer counts a s a mourning day, and there is an outside reason (Rav Shimon Bar Yochai’s death) why we lift the mourning.
(5) Breaks it up – 6 days in Nissan and 27 days in Iyar, and we don’t count Rosh Chodesh and Lag B’Omer.

PRACTICES BY UNDERSTANDING

Rav Moshe continues:

(1) Is not practiced by anyone; even most Sefardim allow haircuts on Lag B’Omer.

(6) Is not quoted by the Rema and the Magen Avraham therefore deduces that it is not an accepted custom.

This leaves (2) on Team A and (3), (4), and (5) on Team B.

Says Rav Moshe, one can change from year to year, within his team – so depending on what’s convenient, can do (3), (4) or (5) changing each year. However one cannot change teams, switching from what’s known as first half to second half.

Now, if I haven’t lost you yet, here’s where it gets tricky. Rav Moshe explains that this is according to the Gra’s understanding. However the Bach understands that even (1) and (2) are only commemorating the 33 days, and therefore Team B now includes (1) through (5) in its minhag.

However, Rav Moshe concludes that generally only Sefardim who ordinarily rule like the Shulchan Aruch should rely on this and allow a cessation of mourning after Lag B’Omer, but Ashkenazim should choose from (3), (4) and (5). Rav Dovid Feinstein Zt”l also paskened that there is no Ashkenazi minhag to only keep aveilus from Pesach through Lag B’Omer.