Pentecost is a term that means fiftieth. This application is the fiftieth day after the Passover. It was the completion of the “feast of weeks” , which started on the third day after the Passover with the offering of the first harvest sheaves to God, and ended with the offering of two loaves of unleavened bread, exhibiting the first fruits of the harvest.
Leviticus 23: 17 (NASB)
17 You shall bring in from your dwelling places two loaves of bread for a wave offering, made of two-tenths of an ephah; they shall be of fine flour, baked with leaven as first fruits to the Lord.
We celebrate Pentecost today as the anniversary of the coming of the Holy Spirit.
Act 2: 40-42 (NASB)
40 And with many other words he solemnly testified and kept on exhorting them, saying, “Be saved from this perverse generation!” 41 So then, those who had received his word were baptized; and that day there were added about three thousand souls. 42 They were continually devoting themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer.