The Omer was an ancient Hebrew measure of grain. Biblical law (Leviticus 23:9-11) forbade any use of the new barley crop until an omer was brought as an offering to the Temple in Jerusalem. The Book of Leviticus (23:15-16) also commanded: "And from the day on which you bring the offering . . . you shall count off seven weeks. They must be complete." This commandment led to the traditional practice of S'firat HaOmer, or "Counting the Omer."
The seven weeks of counting the Omer spans the 49 days between the second day of Passover and the beginning of Shavuot. Thus, S'firat HaOmer links the Exodus from Egypt with the giving of the Torah at Sinai. Jewish mystics expanded upon this historical bond, seeing the period as joining the Jewish people's physical (Pesach) and spiritual (Shavuot) redemption.
After the Temple in Jerusalem was destroyed in 70 C.E., the Omer offering could no longer be observed. But the practice of counting the Omer continued and is still observed by many Jews.
There is a prescribed ritual for counting the Omer. Each evening of the 49 day period, Jews say a special blessing, recite a prescribed formula for counting each day, then read a psalm and a special prayer. Excluding the first verse, the psalm has precisely 49 words.
See also: Counting of the Omer