Lag B'Omer, the 33rd day of the Omer ('Lag' has a numerical value of 33), marks the date of death of one of the greatest Talmudic sages, Rabbi Shimon Bar Yochai. This is a day of great celebration, because tradition says that on his death bed Rabbi Shimon revealed the secrets of the Zohar, the primary book of Jewish mysticism (kabbalah). For centuries, Lag B'Omer has been a day of pilgrimage to the tomb of Rabbi Shimon in the Galilee town of Meiron. In one day, an estimated 250,000 Jews visit Meiron – dancing, praying, and celebrating the wonderful spiritual gifts that Rabbi Shimon bequeathed to us.
To celebrate Lag B'Omer, Jews from around the world light bonfires, to commemorate the great mystical illuminations that Rabbi Shimon revealed. (adapted from aish.com)