Counting the Omer:
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 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 Orthodox Union has a chart that provides the transliterated Hebrew and English text of the counting day-by-day.
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 (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. The word "Lag" is not really a word; it is the number 33 in Hebrew, as if you 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.
List of Dates
Lag B'Omer will occur on the following days of the secular calendar:
Jewish Year 5775: sunset May 6, 2015 - nightfall May 7, 2015
Jewish Year 5776: sunset May 25, 2016 - nightfall May 26, 2016
Jewish Year 5777: sunset May 13, 2017 - nightfall May 14, 2017
Jewish Year 5778: sunset May 2, 2018 - nightfall May 3, 2018
Jewish Year 5779: sunset May 22, 2019 - nightfall May 23, 2019
