Lag ba'Omer is the 33rd day of the counting of the Omer. The count continues for the 49 days between Pesach and Shavuot, and represents the grain offerings that were brought to the Jerusalem Temple every night throughout this period. The period of the Omer is considered to be a time of mourning of varying strictness. However, Lag ba'Omer is celebrated as a special day of release from the prohibitions of the mourning period.
The Talmud (Yevamot 62.2) teaches that Lag was the day in which a great plague that had killed 24,000 students of Rabbi Akiva ended. This day is especially celebrated by rabbinic students, and so is often called the Scholars' Feast. The day is celebrated by Kabbalists as the yahrzeit of Rabbi Simeaon bar Yochai, the author of the Zohar.
On the evening of Lag ba'Omer (which comes before the day, as the day is reckoned to start at sunset), bonfires are often lit. The day itself is often taken up with all the activities that are frowned upon during the other days of the counting of the Omer: dancing, singing and picnics. As weddings are, generally, not celebrated during the counting of the Omer, many couples choose Lag as their wedding day.