The Feast of Harvest (Exodus 23:16) or Feast of Weeks (Deuteronomy 16:10) was one of the three annual festivals, when every male Jew had to appear before the Lord. The others were the Passover and the Feast of Booths. At the Feast of Harvest, the first fruits of the new harvest were brought to the temple as an offering to God (Deuteronomy 16:16-17).
The Feast of Harvest was fifty days after Passover (Leviticus 23:16), and thus became later known as Pentecost, based on the Greek word for fiftieth: πεντηκοστό (pentecosto).
The timing of the outpouring of the Holy Spirit was unique, because this was only one of three occasions during the year when all the religious male Jews came together in Jerusalem. At that time Jews were living in Jerusalem who were born in other countries and therefore had knowledge of other languages. On the first Pentecost after Jesus’ death and resurrection, there were probably also many Jews from neighboring countries, such as Arabia (Acts 2:5,11).