The festival of Shavuot (or Shavuos, in Ashkenazi usage; Shabhuʿoth in Classical and Mizrahi Hebrew Hebrew: שבועות, lit. “Weeks”) is a Jewish holiday that occurs on the sixth day of the Hebrew month of Sivan (late May or early June). Shavuot commemorates the anniversary of the day God gave the Torah to the entire Israelite nation 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.
Shavuot is also Pentecost when Yahweh sent the Holy Spirit to those who belong to Him.
Take the time to review http://www.hebrew4christians.com/Holidays/Spring_Holidays/Shavuot/Shavuot.pdf. For more detail and some interesting facts.