Shavuot

Shavuot marks the giving of the Torah at Sinai, counted as seven weeks after Passover. The custom is to stay up all night studying, called a tikkun leil Shavuot, and to eat dairy, cheesecake and blintzes being the usual suspects. It gets less attention than the holidays with a meal and a story, which is a little unfair given what it's actually commemorating.