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) This was the third month in Israel's calender. It was fulfilled on the "day of Pentecost" as the Feast of Pentecost was occuring, fifty days after Passover - Grk. (Acts.2:1-17) on the same day, just as Passover was fulfilledon the same day as the actual Feast of Passover was being celebrated. God is confirming that He will abide by His calendar that was foreshadowed by the Church in the wilderness.
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 because 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 was begun again with the Christ our Passover. Note the chart above. Three hundred year later (1800 AD.) this was followed by Water Baptism by immersion, (The Laver, Red Sea, Unleavened Bread). 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 completing His calendar for His Bride, the Church. (The Bride the Lamb's wife - Eph.4:11-16; 5:21-33)
Remember: One day with the Lord is as a 1000 years. Therefore it has only been half a day since the Reformation. We MUST see time as God sees it!