Shavuot happens seven weeks and one day after the beginning of Passover.

Exodus 34:22

You shall make the Festival of Weeks with the first offering of the wheat harvest.

Numbers 28:26-31

On the day of the first-fruits, when you offer a new meal-offering to YHVH on your Festival of Weeks, it shall be a holy convocation to you. You shall not do any laborious work.