Lag Baomer is the 33rd day of the omer, the counting of the days between Passover and Shavuot. Set in the Spring it has become a day to enjoy the season with relay races and outdoor sports all culminating in the memorable bonfires.
The counting of the Omer is a mourning period in remembrance of the followers of the great sage of blessed memory, Rabbi Akiva, 24,000 of whom died in a horrible plague in the time between Passover and Shavuot during the 135 C.E. Bar Kochba revolt against the Romans. During the Omer, many Jews follow mourning practices like not shaving, getting married, attending celebrations or cutting their hair.
Lag BaOmer is the yahrzeit or hillula (day of commemoration) for Rabbi Shimon Bar Yochai, one of the best known of Rabbi Akiva’s followers and the writer of the Zohar (the central book of Kabbalah or Jewish mysticism). Before his death, Bar Yochai told his students not to mourn his death and he's been celebrated ever since.
Lag Baomer is in the Spring smack in the middle of Passover and Shavuot, it usually falls in May.
In Israel, many people go to Rabbi Bar Yochai's grave on Mount Meron in the Galilee. There is a belief that if you pray at the Rabbi’s grave, especially on Lag BaOmer, your prayers will be answered. Many families also bring their three-year-old sons to Mt. Meron for a first haircut, also known as an upsherin. Outside of Israel, it is also popular to cut kids' hair.
In and out of Israel, people celebrate happy occasions like weddings, light bonfires, have Israeli singing and dancing, hold sporting competitions and have picnics. In Israel, the most popular foods are very basic, but yummy … wrapping up some potatoes and onions in tinfoil and throwing them into the fire.
Outside of Israel it has become customary to eat S’mores because of all the bonfires.