According to the Torah (Leviticus 23,15), we are obligated to count the days from the second night of Passover to the day before Shavu'ot, seven full weeks. 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.
Every night, from the second night of Passover to the night before Shavu'ot, we recite a blessing and state the count of the omer in both weeks and days. So on the 16th day, you would say "Today is sixteen days, which is two weeks and two days of the Omer".
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.
For many observant Jews, 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 thought to be forbidden. Since such asceticism is more in the spirit of Christian Lent than the Torah and since the Talmud recommends against such practices, we suggest that they be forgotten entirely.
The 33rd day of the Omer (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 stopped on that date. The word "Lag" is not really a word; it is the number 33 in Hebrew, as if one were to call the Fourth of July "Iv July" (IV being 4 in Roman numerals). See Hebrew Alphabet for more information about using letters as numbers.
Lag b'Omer occurs on the following days on the civil calendar:
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