From the Shavuot Wikipedia page:
Shavuot, known as the Feast of Weeks in English and as Pentecost in Ancient Greek, is a Jewish holiday that occurs on the sixth day of the Hebrew month of Sivan (may fall between 14 May–15 June).
Shavuot has a double significance. It marks the all-important wheat harvest in the Land of Israel (Exodus 34:22); and it commemorates the anniversary of the day God gave the Torah to the entire nation of Israel assembled at Mount Sinai, although the association between the giving of the Torah (Matan Torah) and Shavuot is not explicit in the Biblical text.
The holiday is one of the Shalosh Regalim, the three Biblical pilgrimage festivals. It marks the conclusion of the Counting of the Omer, and its date is directly linked to that of Passover. The Torah mandates the seven-week Counting of the Omer, beginning on the second day of Passover, to be immediately followed by Shavuot. This counting of days and weeks is understood to express anticipation and desire for the giving of the Torah. On Passover, the people of Israel were freed from their enslavement to Pharaoh; on Shavuot they were given the Torah and became a nation committed to serving God. The word Shavuot means weeks, and the festival of Shavuot marks the completion of the seven-week counting period between Passover and Shavuot. The yahrzeit of King David is traditionally observed on Shavuot. Hasidic Jews also observe the yahrzeit of the Baal Shem Tov.
Shavuot is one of the less familiar Jewish holidays to secular Jews in the Jewish diaspora, while those in Israel as well as the Orthodox community are more aware of it. According to Jewish law, Shavuot is celebrated in Israel for one day and in the Diaspora (outside of Israel) for two days. Reform Judaism celebrates only one day, even in the Diaspora.