According to tradition, Lag Ba'omer marks the yahrzeit (anniversary of the death) of Rabbi Shimon bar Yochai who was the greatest teacher of Torah in his generation. On the day of his death, he revealed the deepest secrets of the Torah in a Kabbalistic work called the Zohar.

According to the Zohar, Rabbi Shimon's house was filled with fire and light that entire day as he taught his students. At the end of the day, the fire subsided and Rabbi Shimon died. On successive years, his students sought to recreate that experience of light and mystical revelation by kindling bonfires and studying the Zohar in the light of the flames.

Although the anniversary of the death of a tzaddik is usually a mournful day, the anniversary of Rabbi Shimon's death on Lag Ba'omer is a festive one. Bonfires are lit and people sing and dance by the flames. Weddings, parties, listening to music, picnics, and haircuts are commonplace. (an excerpt from Wikipedia)