Lag BaOmer is the 33rd day of the Time of Omer, the days of mourning between Pesach and Shavuot. The Days of Omer are named after a unit of measure for barley (Hebrew: omer). An omer was the measure of barley given daily in sacrifice at the start of the harvest from the second day of Pesach to the start of Shavuot at the time of the Temple in Jerusalem. An omer is thought to have been between 2.2 and 4.4 litres of barley. The days between Pesach and the Festival of Weeks were counted as Omer Days. There are 49 Omer Days: the Festival of Weeks starts on the 50th day.
These seven weeks are days of mourning in memory of the sufferings of the Jews under the Emperor Hadrian and the persecutions they endured at the hands of the Crusaders. For this reason no festivals or weddings are held during this period. The sole exception is the 33rd day, known as Lag BaOmer, since legend has it that on this day an epidemic which had been raging amongst the pupils of the legendary rabbi Akiba suddenly ceased.