Shavuot is both an Israeli national holiday as well as a Biblical holiday.
Shavuot (“weeks”) is also known as the Feast of Weeks or Pentecost, and occurs on the sixth day of the Hebrew month of Sivan, which generally falls sometime between May 15 and June 14. The holiday is significant to the Jewish people for two reasons. First, it signifies the wheat harvest in the Land of Israel. Second, Shavuot is an honorary day commemorating the day when God gave the Torah to Israel at Mount Sinai. Shavuot marks the end of the Counting of the Omer, which is a seven-week period that beings on the second day of Passover. Shavuot is more commonly celebrated in Israel and among orthodox Jews, and less commonly celebrated in the Jewish diaspora.
Shavuot is normally celebrated for one day in Israel under Jewish law, and two days in the diaspora. In reform Judaism, the holiday is celebrated for only one day, both Israel and in the diaspora.