You shall count for yourselves -- from the day after the Shabbat, from the day when you bring the Omer of the waving -- seven Shabbats, they shall be complete. Until the day after the seventh sabbath you shall count, fifty days... You shall convoke on this very day -- there shall be a holy convocation for yourselves -- you shall do no laborious work; it is an eternal decree in your dwelling places for your generations. (Leviticus 21:15-16, Leviticus 21)
Shavu'ot, the Festival of Weeks, is the second of the three major festivals with both historical and agricultural significance (the other two are Passover and Sukkot). Agriculturally, it commemorates the time when the first fruits were harvested and brought to the Temple, and is known as Chag ha-Bikkurim (חַג הַבִּכּוּרִים, the Festival of the First Fruits) or Yom ha-Bikkurim (יוֹם הַבִּכּוּרִים, the Day of the First Fruits). Historically, it celebrates the giving of the Torah at Mount Sinai, and is also known as Z'man Matan Torateinu (זְמַן מַתַּן תּוֹרָתֵנו, the Time of the Giving of Our Torah).
The period from Passover to Shavu'ot is a time of great anticipation. We count each of the days from the second day of Passover to the day before Shavu'ot, 49 days or 7 full weeks, hence the name of the festival: shavu'ot means "weeks." See The Counting of the Omer for more information about the counting. Counting reminds us of the important connection between Passover and Shavu'ot: Passover freed us physically from bondage, but the giving of the Torah on Shavu'ot redeemed us spiritually from our bondage to idolatry and immorality. Shavu'ot is also known as Pentecost, because it falls on the 50th day; however, Shavu'ot has no particular similarity to the Christian holiday of Pentecost, which occurs 50 days after their Spring holiday.
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, not the receiving, that makes this holiday significant.
Shavu'ot is not tied to a particular calendar date, but to a counting from Passover. Because the length of the months used to be variable, determined by observation (see Jewish Calendar), and there are two new moons between Passover and Shavu'ot, Shavu'ot could theoretically occur on Sivan 5th (two 30 day months), 6th (a 29 day month and a 30 day month) or 7th (two 29 day months). However, now that we have a mathematically determined calendar, and the months between Passover and Shavu'ot do not change length on the mathematical calendar, Shavu'ot is always on the 6th of Sivan (the 6th and 7th outside of Israel. See Extra Day of Holidays.)
Work is not permitted during Shavu'ot.
It is customary to stay up the entire first night of Shavu'ot and study Torah, then pray as early as possible in the morning.
It is customary to eat a dairy meal at least once during Shavu'ot. There are varying opinions as to why this is done. Some say it is a reminder of the promise regarding the land of Israel, a land flowing with "milk and honey." According to another view, it is because our ancestors had just received the Torah (and the dietary laws therein), and did not have both meat and dairy dishes available. See Separation of Meat and Dairy. Cheesecake is a popular way of satisfying this tradition. I've posted a simple recipe for a cheesecake-like pie here.
The book of Ruth is read at this time. Again, there are varying reasons given for this custom, and none seems to be definitive.
Shavu'ot will occur on the following days of the secular calendar:
For additional holiday dates, see Links to Jewish Calendars.