Some theories are that if the exact location of Mount Sinai were known, the place would have become a very holy place, and Jerusalem and the Temple could never have become the center of Jewish life. Part of the service on the last day of Passover, Shavuot, and Sukkot is to recite memorial prayers. Av Harachamim, composed for the martyrs, was originally said on the Sabbath before Shavuot. The massacres of Rhineland Jewry many hundreds of years ago also occurred before Shavuot, and the memorial prayers were recited on the Sabbath before. One of the customs observed today is having a Tikkun Leil Shavuot.
People intensely experience the Torah by studying all night.
Some of the other customs of the holiday include eating dairy foods. One theory for this custom suggests the Israelites had no time to prepare heavy meals. It is also said that when the people returned from Sinai, they had the laws of kashrut, and had to make their dishes kosher, so dairy foods were eaten in the meantime. The Book of Ruth is read on the second day to honor the woman who was the great-grandmother of King David. Tradition has it that David was born and died on Shavuot.