What is penetacost?

It is a name used in the Christian Greek Scriptures to denote the Festival of Harvest (Ex 23:16) or Festival of Weeks (Exodus 34:22). It is also called “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. (Leviticus 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.​—Exodus 9:31, 32.

(New World Translation of the Holy Scriptures)