Lag BaOmer is a festive minor holiday that falls during the seven weeks between Passover and Shavuot.
This period of time is known as the Omer. An omer is an ancient Hebrew measure of grain, amounting to about 3.6 liters. 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 Sefirat Ha’omer, or the 49 days of the “Counting of the Omer,” which begins on the second day of Passover and ends on Shavuot. During the time of the counting of the Omer there are bans on parties, music and dancing, similar to the prohibitions for a person in mourning for a loved one.
Lag BaOmer is a shorthand way of saying “the 33rd day of the Omer”. For those that wish to marry in the spring, Lag BaOmer is the only day in which they can celebrate.
Many Jews also do not cut their hair during this time period. Boys, at the age of 3, often have their first haircut on Lag BaOmer, with much festivity surrounding the event.