Lag Ba'omer (ל"ג בעומר) is the 33rd day in the Omer count (ל"ג is the number 33 in Hebrew). The mourning restrictions on joyous activities during the Omer period are lifted on Lag Ba'Omer and there are often celebrations with picnics, bonfires and bow and arrow play by children. In Israel, youth can be seen gathering materials for bonfires.
(Adapted from Wikipedia)