Most people know about Passover. Fewer know that Passover is the opening movement of a seven-week season that culminates in Shavuot.
Passover commemorates the liberation of the Israelites from slavery in Egypt. It begins on the 15th of Nisan and lasts seven days in Israel, eight in the diaspora. The seder is the central ritual. The holiday ends and the counting begins.
On the second night of Passover, Jews begin counting the Omer, a 49-day count to the eve of Shavuot. Each evening there is a blessing and a count. The counting connects the liberation of Passover to the revelation of Shavuot. Passover is the physical liberation; Shavuot is the spiritual one.
The Omer period is a semi-mourning period. Weddings, parties, and haircuts are restricted during most of the count. The reason given is that a plague killed thousands of Rabbi Akiva's students during this period.
Lag B'Omer falls on the 33rd day of the Omer. The mourning restrictions lift for this day. It is celebrated with bonfires and outdoor activities. In Israel, enormous celebrations gather at the tomb of Rabbi Shimon bar Yochai.
Shavuot falls 50 days after Passover and commemorates the giving of the Torah at Mount Sinai. The most distinctive custom is the all-night Torah study session called Tikkun Leil Shavuot. Jews stay awake all night learning and pray the morning service at sunrise. The other universal custom is eating dairy, cheesecake and blintzes on Shavuot are as traditional as matzah on Passover.