Pentecost derives from the Greek word meaning "fifty".
Fifty days from Passover and now 50 days from Christ’s Resurrection. Shavuot (Heb- Pentecost), (OT) Week of Feast, First Fruits, Pentecost. (NT) Birthday of The Church. 50 day after Resurrection, 10 days after His Ascension. Arrival of the of The Holy Spirit.