Lag B'Omer is a holiday celebrated on the 33rd day of the Counting of the Omer. The Omer is a specific interval of time between two other Jewish holidays – Pesach and Shavuot. The counting of the omer begins on the day after Pesach, when a sheaf of barley was sacrificed to the Temple in Jerusalem and ends the day before Shavuot, when a sheaf of wheat was brought to the Temple. The Omer lasts for seven weeks. This year, Lag B'omer falls on May 18th.
The tradition of celebrating Lag B'Omer dates back to the Middle Ages, to the era of the spread of the mystical teachings of Kabbalah. The Bible does not mention this holiday. During this period, many laws were in force, reminiscent of the laws of Jewish mourning. The Talmudic tradition associated this with historical events: under the anti-Roman revolt of Bar Kokhba in the middle of the II century AD many Jews suffered from the plague, which hit the rebels during the Omer period. On the 33rd day of the Omer, the plague stopped, so all mourning restrictions were traditionally lifted on Lag B'Omer. Many weddings were held on this day, classes in heders and yeshivas were canceled, and children went to the forests with their teachers to shoot homemade bows, playing out the battles between the Romans and the Jewish rebels. In Israel, this day is still associated with picnics, barbecues, and field trips, where children bake potatoes in the ashes of a fire.