Lag BaOmer, also Lag B'Omer or Lag LaOmer, is a Jewish religious holiday celebrated on the 33rd day of the Counting of the Omer, which occurs on the 18th day of the Hebrew month of Iyar.

Counting of the Omer is a ritual in Judaism. It consists of a verbal counting of each of the 49 days between the holidays of Passover and Shavuot. The period of 49 days is known as the "omer period" or simply as "the omer" or "sefirah".