When we refer to the Christian feast of Pentecost, it didn’t portray immediately evident of what the word itself means or where it came from.
However, the name for the feast comes directly from the Acts of the Apostles where it says, “When the day of Pentecost had come, they were all together in one place” (Acts 2:1).
Moreover, Pentecost is a Greek word which identifies the conclusion of the Jewish harvest festival.
In other words, the Catholic Encyclopedia notes the other names this feast has in the Old Testament and why Pentecost was used by some Jews.
The term, was adopted from the Greek-speaking Jews (Tob. 2:1; II Mac. 12:32; Josephus, “Ant.”, III, x, 6; etc.) which alludes to the fact that the feast, known in the Old Testament as “the feast of harvest of the firstfruits” (Exodus 23:16), or “the feast of weeks” (Exodus 34:22; Deuteronomy 16:10: 2 Chronicles 8:13), the “day of firstfruits” (Numbers 28:26), and it was called by later Jews ‘asereth or ‘asartha’ (solemn assembly, and which probably means “closing festival,” Pentecost being the closing festival of the harvest and of the Paschal season), fell on the fiftieth day from “the next day after the sabbath” of the Passover (Leviticus 23:11).
Leviticus however, explains how the celebration of this feast is to be counted, (Leviticus 23:15-16) “And you shall count from the morrow after the sabbath, from the day that you brought the sheaf of the wave offering; seven full weeks shall they be, counting fifty days to the morrow after the seventh sabbath; then you shall present a cereal offering of new grain to the Lord”.
This is a term that was carried over from Judaism to Christianity, as it went further to describe the counting of 50 days before its celebration.
The feast of Pentecost for Christians is however, held 50 days after Easter Sunday and it marks the conclusion of the Easter season.