In commemoration of the death of Rabbi Akiva's 24,000 students, we observe 33 days of semi-mourning between Pesach & Shavuot (as the students died for 33 days, until the fever broke). The 3 prevalent customs are:

1) From the start of the Omer (this Sun. Night) until Lag B'Omer; 2) From 2 Iyar until Erev Shavuot; 3) From Rosh Chodesh Iyar until the 3rd day before Shavuot.

Any of the 3 customs are acceptable, & may be changed from year to year if so desired (says Rav Moshe Feinstein).Rav Moshe also rules that guests invited to a wedding (or to the Sheva Brachot following) that falls outside their minhag may attend the Simcha & still maintain their personal count without interruption, as the wedding is a "personal Yom Tov" for Chatan & Kallah, who "draw after them" friends & family, regardless of the Minhag they follow.