Lag BaOmer is a minor, festive holiday that falls on the 33rd day of the seven-week period between Passover and Shavuot, a period of time is known as the Omer. (The numerical value of the Hebrew letter lamed is 30, and the value of gimel is three; lamed and gimel together are pronounced “lahg.”) This holiday gives us a break from the semi-mourning restrictions (no parties or events with music, no weddings, no haircuts) that are customarily in place for some Jewish communities during the Omer.
The Omer has both agricultural and spiritual significance: it marks both the spring cycle of planting and harvest, and the Israelites’ journey out of slavery in Egypt (Passover) and toward receiving the Torah at Mount Sinai (Shavuot).
An omer (“sheaf”) is an ancient Hebrew measure of grain. Biblical law forbade any use of the new barley crop until after an omer was brought as an offering to the Temple in Jerusalem. The Book of Leviticus (23:15-16) also commanded: “And from the day on which you bring the offering…you shall count off seven weeks. They must be complete.” This commandment led to the practice of the S’firat HaOmer, or the 49 days of the "Counting of the Omer,” which begins on the second day of Passover and ends with the celebration of Shavuot on the 50th day.
Lag BaOmer commemorates a variety of historical events, including the end of a plague that killed many students of Rabbi Akiva (c. 50-135 C.E.), the yahrzeit of 2nd-century mystical scholar Shimon bar Yochai, and a Jewish military victory over Roman forces in 66 C.E. In remembrance of these events, some people celebrate with picnics and bonfires. Many couples in Israel choose to get married on Lag BaOmer, and many people also choose to wait until that day to get a haircut or beard trim.