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 a day of special joy and festivity.
NOTE: Each holiday begins and ends at sundown on the days listed.