The Torah commands us to count 50 days, beginning with bringing the sheaves of barley on the 16th of Nissan, and culiminating with bringing the two loaves from wheat on the Shavuot holiday.
As it is written (Leviticus 23, 15-16):
"And ye shall count unto you from the morrow after the day of rest, from the day that ye brought the sheaf of the waving; seven weeks shall there be complete;
even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the LORD."
Shavuot has no definitive date in the Torah. Only the counting of the Omer enables us to determine the exact time. Why is there no date for Shavuot? The Chizkuni explains that if there was a set date, we may not be so careful with the counting, but rather simply depend on the date of the holiday. Since the actual counting is so important and "great things depend on its counting", the Torah made certain that the counting be observed meticulously.
Why is it so important to count the Omer? The Ramban explains that the purpose of counting is to connect Pesach and Shavuot and to create one continuous holiday over 49 days. Pesach is the first day of Yom Tov, Shavuot is the last day of Yom Tov and the interim days are like Chol Hamoed.
Why is it so important to connect Pesach and Shavuot?
However, the Torah does not explicitly mention this spiritual viewpoint. It mentions only the agricultural aspect. The counting of the Omer is presented as a process that begins with bringing the sheaves of barley on Pesach, and culminates with bringing the two loaves of wheat on Shavuot. In other words, this is a process related to various crops. In addition, there is a progression from barley – animal feed – to wheat suited for human consumption. In any case, this process reflects the material side of counting the Omer and the expression of gratitude – not the transition from the exodus to the giving of the Torah.
This is a statement of great importance. The Torah does not merely discuss the material, agricultural side per se, but rather the crops of Eretz Yisrael in general. In the Land of Israel, there is spiritual meaning even to agriculture and farming. Agriculture in Israel emanates from a wellspring of holiness. It is related in its very essence to the role and place of Am Yisrael. The celebration of the new harvest is a celebration for the entire people (and therefore overrides Shabbat.)
This joyous period, therefore, connects the physical exodus to receiving the Torah on Shavuot. It connects our gratitude for the agriculture Hashem gave us in our Land with the understanding that the agriculture in our Land emanates from a wellspring of holiness. It symbolizes our transition from an aggregate of individuals to the building of a nation – a status which endows us with the privilege of receiving the Torah.
Counting the Omer is not simply an external act. We also progress each day through the process we have described. B’ezrat Hashem, may we merit to move forward through these days of counting, in particular, and during the course of the year, in general – stage after stage. May we merit to build our spiritual world, our world of positive "midot" (traits), and the world of Torah and Halacha.