If you go back and read the Old Testament, you will discover that Pentecost was one of the Jewish feast days. Only they didn't call it Pentecost. That's the Greek name. The Jews called it the Feast of Harvest or the Feast of Weeks. It is mentioned five places in the first five books—in Exodus 23, Exodus 24, Leviticus 16, Numbers 28, and Deuteronomy 16. It was the celebration of the beginning of the early weeks of harvest. In Palestine there were two harvests each year. The early harvest came during the months of May and June; the final harvest came in the Fall. Pentecost was the celebration of the beginning of the early wheat harvest, which meant that Pentecost always fell sometime during the middle of the month of May or sometimes in early June.
There were several festivals, celebrations, or observances that took place before Pentecost. There was Passover, there was Unleavened Bread, and there was the Feast of Firstfruits. The Feast of Firstfruits was the celebration of the beginning of the barley harvest. Here's the way you figured out the date of Pentecost. According to the Old Testament, you would go to the day of the celebration of Firstfruits, and beginning with that day, you would count off 50 days. The fiftieth day would be the Day of Pentecost. So Firstfruits is the beginning of the barley harvest and Pentecost the celebration of the beginning of the wheat harvest. Since it was always 50 days after Firstfruits, and since 50 days equals seven weeks, it always came a "week of weeks" later. Therefore, they either called it the Feast of Harvest or the Feast of Weeks.
There are three things you need to know about Pentecost that will help you understand Acts 2.
1. Pentecost was a pilgrim festival. That meant that according to Jewish Law, all the adult Jewish men would come from wherever they were living to Jerusalem and personally be in attendance during this celebration.
2. Pentecost was a holiday. No servile work was to be done. School was out. The shops were closed. It was party time.
3. There were certain celebrations and sacrifices and offerings which were prescribed in the Law for the day of Pentecost. On Pentecost, the High Priest was to take two loaves of freshly baked wheat bread and offer them before the Lord. The wheat bread was made from the newly harvested wheat.
In short, Pentecost in the time of the Apostles was a great and grand harvest celebration. The streets of Jerusalem were clogged with thousands of pilgrims who had come from every point of the compass to celebrate the goodness of God and the bringing in of the wheat harvest.
Excerpted from "The P.U.I.H." from Keep Believing Ministries (used by permission).