And thou shalt observe the feast of weeks, even of the first-fruits of wheat harvest, and the feast of ingathering at the turn of the year. (Ex. 34:22)
Seven weeks shalt thou number unto thee; from the time the sickle is first put to the standing corn shalt thou begin to number seven weeks. And thou shalt keep the feast of weeks unto the LORD thy God after the measure of the freewill-offering of thy hand, which thou shalt give, according as the LORD thy God blesseth thee. (Dt. 16:11)
Three times thou shalt keep a feast unto Me in the year. The feast of unleavened bread shalt thou keep; seven days thou shalt eat unleavened bread, as I commanded thee, at the time appointed in the month Abib--for in it thou camest out from Egypt; and none shall appear before Me empty; and the feast of harvest, the first-fruits of thy labours, which thou sowest in the field; and the feast of ingathering, at the end of the year, when thou gatherest in thy labours out of the field. Three times in the year all thy males shall appear before the Lord GOD. (Ex. 23:14-17)
Also in the day of the first-fruits, when ye bring a new meal-offering unto the LORD in your feast of weeks, ye shall have a holy convocation: ye shall do no manner of servile work; but ye shall present a burnt-offering for a sweet savour unto the LORD: two young bullocks, one ram, seven he-lambs of the first year; and their meal-offering, fine flour mingled with oil, three tenth parts for each bullock, two tenth parts for the one ram, a several tenth part for every lamb of the seven lambs; one he-goat, to make atonement for you. Beside the continual burnt-offering, and the meal-offering thereof, ye shall offer them--they shall be unto you without blemish--and their drink-offerings. (Nm. 28:26-31)
This evening, if I'm not mistaken, begins Shavuot, with Sukkot one of the two major harvest festivals of the Jewish calendar (Shavuot is to grain harvest as Sukkot is to fruit harvest). The most common name in English is the Feast of Weeks; it was one of the three pilgrimage holy days of the calendar (the other two being Passover and Sukkot, mentioned above). It's somewhat unusual out of Jewish holy days, though, in that it has only ceremonial commandments associated with it; but this is perhaps fitting insofar as it is traditionally the day God gave Moses the whole covenant at Sinai. The book of the Bible that is associated with it is the book of Ruth, and the feast is also by later tradition associated with King David, being a commemoration of his birth and death. Harvest, Covenant, Redemption, Throne -- there are more important Jewish holy days, but none that sum up so many of the hopeful and joyful features of Jewish history.
It is from Shavuot that Christians get the name 'Pentecost'. Shavuot is calculated by counting seven weeks from the second day of Passover (hence the name 'Feast of Weeks'). Because of this Hellenistic Jews called it Pentecost, i.e., the Fiftieth, and Christianity, which in its ritual roots is nothing other than Messianic Hellenistic Judaism opened to Gentiles, carried over the holiday and name, associated now with the Descent of the Spirit on the Church; but as the calendars diverged, so did the holidays.