An omer refers to an ancient Hebrew measure of grain, amounting to about 3.6 litres. Biblical law forbade any use of the new barley crop until 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 forty-nine days of the "Counting of the Omer". The omer is counted from the second day of Passover and ends on Shavuot.
Lag Ba'Omer is the shorthand way of saying the thirty-third day of the omer. It is celebrated to commemorate the day a plague ended in which thousands of students of Rabbi Akiba, a Talmudic scholar, died during the Counting of the Omer. The period of counting is traditionally observed as a period of mourning. The mourning, however, is set aside on Lag Ba'Omer, making it day of special joy and festivity.