What was the Day of Pentecost?

Pentecost is a Greek word (Πεντηκοστῆς) for the Israelite Feast of Harvest (introduced to the Israelites at the time of Moses in Exodus 23:16).

“Pentecost was the celebration of the beginning of the early weeks of harvest. In Palestine, there were two harvests each year. The early harvest came during the months of May and June; the final harvest came in the Fall. Pentecost was the celebration of the beginning of the early wheat harvest, which meant that Pentecost always fell sometime during the middle of the month of May or sometimes in early June.” (Source)