Counting of the Omer
The period between Passover and Shavuot is called the “Counting of the Omer” (Sefirat Ha'omer). Omer means "barley sheaf" and refers to the offering brought to the Temple on the second day of Passover. Starting from that day, the Torah also instructs that “you shall count off seven weeks. They must be complete: you must count until the day after the seventh week — 50 days” (Leviticus 23:15-16).
The festival of Shavuot is observed on the 50th day. According to the Bible (Leviticus 22:15), "You shall count for yourselves from the day after the day of rest, from the day brought the Omer of the wave offering; seven complete weeks there shall be, until the day after the seventh week shall you number 50 days..."
The counting, which lasts for 49 days, takes place at night, commencing with the second night of Pesach. Those who "count the Omer" recite a blessing each night of the 50-day period:
Baruch atah Adonai Eloheinu melech ha’olam asher kid’shanu b’mitzvotav vitzivanu al sefirat ha’omer.
Blessed are You, Adonai our God, Ruler of the universe, who sanctifies us with mitzvot, and commands us concerning the counting of the Omer.
The Jewish Home (Revised Edition) by Daniel B. Syme