Shavout commemorates God giving the Torah to Moses on Mt Sinai 49 days after the exodus from Egypt. Having fled from oppression and slavery, on that day the Israelites became a nation pledged to serving God.
As a harvest festival it marks the end of the 49 days of Omer, beginning the first day after Pesach. It also marks the day of the wheat harvest. It is referred to in the Torah as the Festival of Weeks (Ḥag ha-Shavuot - Exodus 34:22, Deuteronomy 16:10). As a harvest festival it is also known as the Festival of Reaping (Ḥag ha-Katsir - Exodus 23:16) or Day of the First Fruits ( Yom ha-Bikkurim - Numbers 28:26).
So the festival celebrates both the giving of the Torah and the end of the grain harvest. At the time of the Temple it was also the day on which the first fruits (Bikkurim) – wheat, barley, grapes, figs, pomegranates, olives, and dates (Deuteronomy 8:8) were brought and presented to the priests.
Several special customs are observed on Shavuot. Reading of the Book of Ruth reminds us that we too can find a continual source of blessing in our tradition. Another tradition includes staying up all night to study Torah and Mishnah, a custom called Tikkun Leil Shavuot, which symbolizes our commitment to the Torah, and that we are always ready and awake to receive the Torah. Traditionally, dairy dishes are served on this holiday to symbolize the sweetness of the Torah, as well as the 'land of milk and honey'.