And thou shalt keep the feast of weeks unto the Lord thy God with a tribute of a freewill offering of thine hand, which thou shalt give unto the Lord thy God, according as the Lord thy God hath blessed thee:
Three times in a year shall all thy males appear before the Lord thy God in the place which he shall choose; in the feastof unleavened bread, and in the feastofweeks, and in the feastof tabernacles: and they shall not appear before the Lord empty:
Even after a certain rate every day, offering according to the commandment of Moses, on the sabbaths, and on the new moons, and on the solemn feasts, three times in the year, even in the feastof unleavened bread, and in the feastofweeks, and in the feastof tabernacles.
And ye shall count unto you from the morrow after the sabbath, from the day that ye
brought the sheaf of the wave offering; seven sabbaths shall be complete:
Even unto the morrow after the seventhsabbath shall ye number fifty days; and ye shall offer a new meat offering unto the Lord.
and then you can continue to read Leviticus 23: 17-20
What is Pentecost? Pentecost is significant in both the Old and New Testaments. “Pentecost” is actually the Greek name for a festival known in the Old Testament as the Feast of Weeks (Leviticus 23:15; Deuteronomy 16:9). The Greek word means “fifty” and refers to the fifty days that have elapsed since the wave offering of Passover. The Feast of Weeks celebrated the end of the grain harvest.
You can see that Pentecost was also observed in the New Testament:
And when the day of Pentecost was fully come, they were all with one accord in one place.
For Paul had determined to sail by Ephesus, because he would not spend the time in Asia: for he hasted, if it were possible for him, to be at Jerusalem the day of Pentecost.
But I will tarry at Ephesus until Pentecost.