During the 50-day period from Passover until Shavuoth (the Feast of Weeks), we count sheaves of wheat (omer in Hebrew). This is treated as a somber period. Weddings are not performed, for example. However, the 33rd day of Counting the Omer is a respite from the solemnity; it is a joyful time.
Lag (Hebrew for 33) B'Omer is a minor holiday. It is also the anniversary of the death or Rabbi Shimon Bar Yocahi, the author of the Zohar, which is the basic text on Jewish mysticism called Kabbalah. In Israel, Lag B'Omer is celebrated with parades, bonfires, and a pilgrimage to the grave of Bar Yochai in Meron, Israel.