The Feast of Weeks is called Shavuot, but biblically, this term technically refers to the preceding 7 weeks. The festival itself is called Yom haBikurim, the day of firstfruits. This can be somewhat confusing, because there are two Yomim haBikurim, the early and later First Fruits. The early FirstFruits marks the beginning of counting the 50 days toward the later FirstFruits, which is why later one is also called #pentecost, Greek for fifty.
Numbers 28:26-31

I've made a few videos on the Biblical holiday of #shavuot.