Lag BaOmer (also called Lag B'Omer) is a Jewish holiday that falls on the 18th day of the Hebrew month of Iyar and the 33rd day of the Counting of the Omer. This is a 49-day period that begins on the second day of Passover representing the days in which Omer, a measurement of barly, was offered in the Temple of Jerusalem.
Traditionally, people count down each day of the Counting of the Omer while reciting a prayer. This a period of mourning during which many types of celebrations are forbidden by Jewish law. Haircuts, shaving, instrumental music, and dancing are a few examples of things that are forbidden. Lag BaOmer, on the 33rd day lifts all of these bans for one day, so it is a time for great celebration. Many Jewish couples choose to get married on this day and many boys, after their third birthday, have their first haircut. Bonfires are a common theme on Lag BaOmer. Many people have bonfire parties with family and friends to commemorate the death of Rebbi Shimon, who used fire and light to teach students about the Torah.