Pentecost is an Israelite-Jewish festival. In Exodus 23:14-17 it is called simply the harvest festival, the feast of first-fruits of the grain harvest. In Exodus 34:22 it is called the feast of weeks, the first-fruits of the grain harvest. In Leviticus 23:15-21 the feast is reckoned by counting seven weeks from the beginning of the grain harvest; it is a day of Sabbatical observance. In Numbers 28:26-31 it is called the feast of weeks, the day of first-fruits. In Deuteronomy 16:9-12 it is the feast of weeks, which occurs seven weeks after the beginning of the grain harvest. It is one of the three major festivals in all the older lists of feasts.
It is probable that it was later in origin than Passover and did not take form until the Israelites had become a primarily agricultural community in Canaan. The time of the festival in its original celebration must have been indefinite, since the beginning of the grain harvest can’t be put at a certain day in the calendar. The beginning of the grain harvest corresponds with the feast of Matzoth (unleavened bread). When Passover and Matzoth were combined and set on the 14th of Nisan, the festival of weeks received a regular date in the calendar seven weeks (fifty days) after Passover.
As a major feast, all Jewish males over the age of twelve were expected to try to celebrate it in Jerusalem.