Counting of the Omer
The fifty day period between the Feast of Pesach(Passover) and the Feast of Shavuot(Feast of Weeks) is known as Sephirat HaOmer, the counting of the Omer. On the second day of Pesach, the Omer offering from the new barley crop was brought into the Temple in Jerusalem. It began a period of counting of the Omer and preparation for Shavuot, the anniversary of the giving of the Torah and the yearly celebration of re-accepting the Torah upon ourselves. There is actually a mitzva (a command) to count each specific day which is done at the completion of Arbit, the evening service.
For your convenience, below is the link for the blessings of Sefirat Ha-Omer and also keeping track of counting the Omer :