“When the day of Pentecost had come, they were all together in one place. And suddenly a sound came from heaven like the rush of a mighty wind, and it filled all the house where they were sitting. And there appeared to them tongues as of fire, distributed and resting on each one of them. And they were all filled with the Holy Spirit and began to speak in other tongues, as the Spirit gave them utterance…
…And Peter said to them, “Repent, and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children and to all that are far off, every one whom the Lord our God calls to him.” And he testified with many other words and exhorted them, saying, “Save yourselves from this crooked generation.”
So those who received his word were baptized, and there were added that day about three thousand souls. And they devoted themselves to the apostles' teaching and fellowship, to the breaking of bread and the prayers.”
– Acts 2:1-4, 38-42
What is Pentecost?
The Feast of Pentecost, celebrated the fiftieth day after Easter, commemorates the descent of the Holy Spirit on the Apostles and the birth of the Church. It is celebrated on the day of the ancient Jewish festival called the “Festival of Weeks,” also called Pentecost (and the source of the name of the Catholic feast). The Catholic feast of Pentecost has also been known as Whitsunday, from “White Sunday,” in reference to the white garments worn by those being baptized on the feast day.
Historically, this feast dates back to the first century and the days of the Apostles, but it appears that early on, being so closely bound to Easter, it was not observed as much more than the end of Paschaltide. The reference to Pentecost in Paul's first letter to the Corinthians likely refers to the older, established Jewish feast. The day as a celebration of the Holy Spirit coming down upon the Apostles was well established by the time of the Ecclesiastical writer Tertullian of the second and third century. However, for some time the duration varied; some fourth century writings describe Pentecost as lasting one week, though it was not kept with an octave, but only as a single day, in the west until a much later date.
– Adapted From the Catholic Encyclopedia