According to the Torah (Lev. 23:15), we are obligated to count the days from Passover to Shavu'ot. This period is known as the Counting of the Omer. An omer is a unit of measure. On the second day of Passover, in the days of the Temple, an omer of barley was cut down and brought to the Temple as an offering. This grain offering was referred to as the Omer.

Every night, from the second night of Passover to the night before Shavu'ot, we count of the omer in both weeks and days. The counting is intended to remind us of the link between Passover, which commemorates the Exodus, and Shavu'ot, which commemorates the giving of the Torah. It reminds us that the redemption from slavery was not complete until we received the Torah.

This period is a time of partial mourning, during which weddings, parties, and dinners with dancing are not conducted, in memory of a plague during the lifetime of Rabbi Akiba. Haircuts during this time are also forbidden. The 33rd day of the Omer, or "Lag", which is the Hebrew number for 33, (the eighteenth of Iyar) is a minor holiday commemorating a break in the plague. The holiday is known as Lag b'Omer. The mourning practices of the omer period are lifted on that date, and many people have weddings, bonfires, and sing and dance in joy.