We count 7 weeks from the second day of Pesach. When we get to the 50th day it is called Shavuot. That is actually when the Torah was given to us by Har Sinai. This is one of the 3 "Regalim"-Legs. The three are= 1)Pesach. 2)Shabuot. 3)Sukkot. There is a custom to learn all night, the night of Shabuot. It is also customary to eat only dairy on this holiday.