Lag BaOmer is a festive minor holiday that falls during the seven weeks between Passover and Shavuot (usually in May or June on the Gregorian calendar). 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. Lag BaOmer is a shorthand way of saying “the 33rd day of the Omer”.