The Feast of Pentecost (called in Hebrew Shavuot, Weeks), is also called in English the Feast of Weeks. It is celebrated after counting a week of weeks (seven weeks) from the Feast of Firstfruits.
Annually it is the feast of the summer harvest, just as Firstfruits is the feast of the spring harvest.
The prophetic signifance of the feast was fulfilled when the Holy Spirit came down on the 120 disciples in the Upper Room (Acts 2:1-4).
The way the Holy Spirit descended, in a strong rushng wind, and with flames of fire, was a reminder of when God came down on Mount Sinai with strong rushing winds, and flames of fire.