The Feast of Pentecost adds to the calendar for the Bride of Christ. This feast was celebrated 50 days after the Feast of Passover(Lev.23:15-22) in the third month in Israel's calendar. It was fulfilledhistorically "when the day of Pentecost had fully come" (Acts.2:1). In the City of Jerusalem, the fulfillment "fully came" the same day as the Old Testament Feast of harvest / weeks (50 days after Passover) was celebrated. God is confirming that He is abiding by His calendar that was foreshadowed (Col.2:16-17)by the Church in the wilderness. This is why "there were dwelling at Jerusalem Jews, devout men, out of every nation" (Acts.2:5) to celebrate this Old Testament feast(Deut.12:1-12).
These Feast days / Festival days / Holy days "are a shadow of things to come" (Col.2:16-17)
Note:There was yet another Feast to be celebrated in Israel's calendar,the Feast of Tabernacles. However, the early first century Church never experienced the fulfillment of this Feast.
By about 100 AD? they began to spiritually decline and lost all the truth that the Holy Ghost had lead (John.16:12-13) them into. This ended in the "dark ages"(500 AD) and lasted for about a 1000 years. This darkness was broken by Martin Luther and the Reformation. (1517 AD) and God's calendar of the Feasts began to be restored with the truths of Christ our Passover(1Cor.5:7-8). This parallel’s Israel being delivered by the bondage of Egypt. Three hundred years later (1522 - 1800 AD?) this was followed by Water Baptism by immersion. (This parallel's the Laver, Red Sea, and the feast of Unleavened Bread Review: Phase. 2 and 4). Then one hundred years after that (1906 AD), God poured out His Spirit as He did on the day of Pentecost. (The Feast of Pentecost) For the last 100 years God has been leading the Church toward the Feast of Tabernacles to complete His calendar for His Bride, the Lamb's wife (Eph.4:11-16; 5:21-33).