Feast of Pentecost
Descent of the Holy Spirit (Fifty Days after Pascha (Easter))
"We have seen the true light, we have received the Heavenly Spirit, we have found the true faith, we worship the undivided Trinity, for it has saved us."
Just before the Ascension of Our Lord, Jesus told the Apostles "I am sending the promise of my Father upon you; but stay in the city until you are clothed with power from on high" (Luke 24:49), for "John baptized with water, but in a few days you will be baptized with the Holy Spirit." (Acts 1:5)
In the Old Testament, the Jews celebrated the Feast of Harvest on the fiftieth day after the Passover (Exodus 23:16), which later Greek speaking Jews referred to simply as 'Pentecost' (or '50th day'). The first Christians retained the name Pentecost, but for them, the feast occurred 50 days after the Pasch (Resurrection).
After the Ascension, the Apostles returned from Mount Olivet to the Upper Room, where they spent time in continuous prayer (Acts 1:13-14). "When the time for Pentecost was fulfilled, they were all in one place together. And suddenly there came from the sky a noise like a strong driving wind, and it filled the entire house in which they were. Then there appeared to them tongues as of fire, which parted and came to rest on each one of them. And they were all filled with the Holy Spirit and began to speak in different tongues, as the Spirit enabled them to proclaim.
Now there were devout Jews from every nation under heaven staying in Jerusalem. At this sound, they gathered in a large crowd, but they were confused because each one heard them speaking in his own language. They were astounded, and in amazement they asked, 'Are not all these people who are speaking Galileans? Then how does each of us hear them in his own native language?'" (Acts 2:1-8)