There are two customs for observing sefira, the period of semi-mourning between Pesach and Shavuot.
Custom A
The mourning observances begin on the first night of Sefira, and end on Lag B'omer, the 33rd day of Sefira.
Custom B
The mourning observances begin on Rosh Chodesh Iyar, and end on Shavuot. On Lag B'omer, the 33rd day of Sefira, the mourning is suspended.
Custom A never made much sense to me. It requires mourning during Nissan (the happiest month of the year (TM)) and the message of Lag B'omer is hard to understand. I mean, if you keep the first custom, what exacty are you celebrating on Lag B'omer? The end of the plauge which anihilated Rabbi Akiva's school? Mazal tov: Of course the plague ended: Everyone was dead! It doesn't seem, to me, like a reason for parties.
The second custom is more sensible. If you follow it, you won't mourn in Nissan (THMOTY) and Lag B'omer is not the very end of the plauge, but a day when the plague, like our mourning observances, was suspended.
Naturally, those of you who follow the Jewish world will not be surprised to learn that the sensible approach has fallen out of favor. Everyone I know follows custom A, and seems oblivious to the inconsistancies.