The Jewish holiday of Shavuot begins tomorrow (May 26) at sundown and lasts until Monday, May 28th.
This ancient holiday originally marked the beginning of the agricultural season, but after the destruction of the Temple in 70 CE the rabbis connected Shavuot with the Revelation at Mt. Sinai. Hence, in modern times it celebrates God giving the Torah to the Jewish people. Some Jews stay up all night studying the Torah in honor of this event. Other customs include chanting the Ten Commandments and enjoying dairy foods such as cheese and cheesecake.
Learn more about Shavuot and its customs in: