Counting of the Omer (Hebrew: Sefirat HaOmer, sometimes abbreviated as Sefira or the Omer) is an important verbal counting of each of the forty-nine days between the Jewish holidays of Passover and Shavuot as stated in the Hebrew Bible: Leviticus 23:15–16.

This mitzvah ("commandment") derives from the Torah commandment to count forty-nine days beginning from the day on which the Omer, a sacrifice containing an omer-measure of barley, was offered in the Temple in Jerusalem, up until the day before an offering of wheat was brought to the Temple on Shavuot. The Counting of the Omer begins on the second day of Passover (the 16th of Nisan) for Rabbinic Jews (Orthodox, Conservative, Reform), and after the weekly Shabbat during Passover for Karaite Jews, and ends the day before the holiday of Shavuot, the 'fiftieth day.'

The idea of counting each day represents spiritual preparation and anticipation for the giving of the Torah which was given by God on Mount Sinai at the beginning of the month of Sivan, around the same time as the holiday of Shavuot. The Sefer HaChinuch (published anonymously in 13th century Spain) states that the Hebrew people were only freed from Egypt at Passover in order to receive the Torah at Sinai, an event which is now celebrated on Shavuot, and to fulfill its laws. Thus, the Counting of the Omer demonstrates how much a Hebrew desires to accept the Torah in his own life. 

~ Courtesy of https://en.wikipedia.org/wiki/