The word Pentecost is from the Greek word meaning fiftieth. It’s the fiftieth day after Jesus‘ death, when the Holy Spirit can to earth, as Jesus had promised (John 14:26) to comfort his followers and be their counselor (Acts 2).
Although the word Pentecost is only used in the New Testament, there was also a feast on that same day in the Old Testament. It was called “the feast of the harvest” or “the day of first fruits” (Exodus 23:16 and Numbers 28:26).