Shavuot or the Festival of Weeks, commemorates the time when the first fruits were harvested and brought to the Temple and it celebrates the giving of the Torah at Mount Sinai. The period between Passover and Shavuot is a time of great anticipation.
We count each of the days from the second day of Passover to the day before Shavuot, forty-nine days or seven full weeks, hence the name of the festival. The counting of forty-nine days between Passover and Shavuot reminds us of the important connection between these two holidays. Passover freed us physically from bondage, but the giving of the Torah on Shavuot redeemed us spiritually from our bondage to idolatry and immorality.
It is noteworthy that the holiday is called the time of the giving of the Torah, rather than the time of the receiving of the Torah. The Sages point out that we are constantly in the process of receiving the Torah, that we receive it every day, but it was first given at this time, thus it is the giving and not the receiving that makes this holiday significant.
It is customary to stay up the entire first night of Shavuot and study Torah, then pray as early as possible in the morning. It is also a ritual to eat a dairy meal at least once during Shavuot.