How to Count the Omer
The omer refers to the forty-nine day period between the second night of Passover (Pesach) and the holiday of Shavuot. This period marks the beginning of the barley harvest when, in ancient times, Jews would bring the first sheaves to the Temple as a means of thanking God for the harvest. The word omer literally means "sheaf" and refers to these early offerings.
The Torah itself dictates the counting of the seven weeks following Pesach:
"You shall count from the eve of the second day of Pesach, when an omer of grain is to be brought as an offering, seven complete weeks. The day after the seventh week of your counting will make fifty days, and you shall present a new meal offering to God (Leviticus 23:15-16)."
In its biblical context, this counting appears only to connect the first grain offering to the offering made at the peak of the harvest. As the holiday of Shavuot became associated with the giving of the Torah, and not only with a celebration of agricultural bounty, the omer period began to symbolize the thematic link between Peach and Shavuot.
While Pesach celebrates the initial liberation of the Jewish people from slavery in Egypt, Shavuot marks the culmination of the process of liberation, when the Jews became an autonomous community with their own laws and standards. Counting up to Shavuot reminds us of this process of moving from a slave mentality to a more liberated one.
When to Count the Omer
The counting of the omer begins on the second night of Pesach. Jews in the Diaspora generally integrate this counting into the second seder.
The omer is counted each evening after sundown. The counting of the omer is generally appended to the end of Ma'ariv (the evening service), as well.
What to Say. . . and What Not to Say
One stands when counting the omer, and begins by reciting the following blessing:
Barukh ata Adonai Eloheinu Melekh ha'Olam asher kid'shanu b'mitzvotav v'tizivanu al sefirat ha'omer.
Blessed are you, Adonai our God, Sovereign of the Universe, who has sanctified us with your commandments and commanded us to count the omer.