Pentecost is the liturgical season after Easter which celebrates the descent of the Holy Spirit upon the Church, ushering in a new era for the people of God. In Acts 2:1-41 the Holy Spirit descended upon apostles in which they were able to speak their native Galilean dialect but were understood by people of many different languages and nations. Pentecost begins the eighth Sunday, or 50 days, after Easter Sunday.
“Pentecost” is from Greek meaning “fiftieth.” The name Pentecost was originally given to the Jewish Feast of Weeks which fell on the fiftieth day after Passover, when the first fruits of the grain harvest were offered to the Lord (Leviticus 23:15-21 and Deuteronomy 16:9-11). The second chapter of Acts begins by noting that the Feast of Weeks had just passed.
Some people regard it as the birthday of the Church because from that point on the apostles carried the message of Christ to the whole world. It is indeed a new age of the Church through which Christ works in different means for our salvation.
In this age of the Church Christ now lives and acts in and with his Church, in a new way appropriate to the new age. He acts through the sacraments [...]; this is the communication of the fruits of Christ’s Paschal mystery in the celebration of the Church’s ‘sacramental’ liturgy.
Catechism of the Catholic Church, paragraph 1076.
In other words, in the absence of Christ’s physical presence to us due to his fulfillment of Scripture by his death and resurrection, Jesus has left us the sacraments through which the Holy Spirit conveys the grace of God in a physical and tangible way. Pentecost marks the beginning of this new, sacramental era in which we live today.
The liturgical color for Pentecost is red.