What was the significance for christianity of the first pentecost after the resurrection of jesus?

Pentecost is the name used in the Christian Greek Scriptures to denote the Festival of Harvest (Ex 23:16) or Festival of Weeks (Ex 34:22), called also “the day of the first ripe fruits.” (Nu 28:26) Instructions for this festival are found at Leviticus 23:15-21; Numbers 28:26-31; Deuteronomy 16:9-12. It was to be celebrated on the 50th day (Pentecost means “Fiftieth [Day]”) from Nisan 16, the day that the barley sheaf was offered. (Le 23:15, 16) In the Jewish calendar it falls on Sivan 6. It was after the barley harvest and the beginning of the harvest of wheat, which ripened later than the barley.

It was on the day of Pentecost that the holy spirit was poured out by Jesus Christ on the group of about 120 disciples in the upper room at Jerusalem in the year 33 C.E. (Ac 1:13-15) Jesus had been resurrected on Nisan 16, the day of the offering of the barley sheaf by the high priest. He was, in a figurative sense, without leaven, which represents sin. (Heb 7:26)