On the second day of Passover 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 (Pentecost), there is a blessing
Baruch atah A-donai E-loheinu Melekh Ha-olam asher kid’shanu b’mitzvotav v’tzivanu al S’firat Ha-omer. (“Blessed are You, Lord our God, King of the Universe, Who has sanctified us with His commandments and commanded us to count the Omer.”)
and the count of the omer in both weeks and days. On the 20th day, for example, one says “Today is twentieth days, which is two weeks and six days of the Omer.”
The counting connects Passover and Shavu’ot (Pentecost – which commemorates the giving of the Torah – the central scriptures). Redemption from slavery is not complete until the reception of the Torah.
15 And from the day after the sabbath, from the day on which you bring the sheaf of the elevation offering, you shall count off seven weeks; they shall be complete.
16 You shall count until the day after the seventh sabbath, fifty days; then you shall present an offering of new grain to the LORD.
9 You shall count seven weeks; begin to count the seven weeks from the time the sickle is first put to the standing grain.
10 Then you shall keep the festival of weeks for the LORD your God, contributing a freewill offering in proportion to the blessing that you have received from the LORD your God.