A measure of barley called the "Omer" was offered on the second day of Passover, marking the first day of a fifty day season where each day is counted. The festival of Shavuot occurs on the fiftieth day. During this season, it is recounted in the Talmud that the students of Rabbi Akiva died in a plague because they did not give each other proper respect. Some manuscripts recount the plague ending on Lag B'Omer--the thirty-third day of the fifty. Kabbalistic tradition recounts that the great sage Rabbi Shimon Bar Yochai died on Lag B'Omer and that the sun miraculously refused to set until he expired, hence the Chasidic tradition of candles and bonfires on Lag B'Omer.

In modern day Israel, Lag B'Omer campfires are pervasive among both traditional and secular Jews. Tens of thousands of Chasidic Jews make a pilgrimage to Rabbi Shimon Bar Yochai's grave at Mt. Meron, where huge bonfires speckle the landscape.

Resources:
Learn about the author of the Zohar:
A Text Study: Rabbi Shimon bar Yochai (PDF file 84Kb)
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D'var Torah: How Should I Love Thee, Let Me Count the Days