The word “Pentecost” comes from the Greek word meaning “fiftieth” and refers to the Jewish festival celebrated on the fiftieth day after the Passover Sabbath Feast.  According to Acts 2:1-21, the Holy Spirit is the agent whereby the risen Christ is made present to the Church during the Jewish festival of Pentecost.

The “Day of Pentecost” is the closing of the annual Easter festival.  The formation of the church is an immediate consequence of the resurrection, not a delayed reaction to it.  And, because the church is built upon the resurrection, the festival of the resurrection (Eastertide) must culminate in the affirmation of the birth of the church of the Day of Pentecost.  The church cannot wait until some time later in the liturgical year to affirm the church as a community of faith.  The community’s faith is a faith in the risen Christ.  Thus, the Day of Pentecost is the appropriate culmination of Eastertide.
The liturgical color for the Day of Pentecost (and for the following days of the week) is red.  Red is the color of fire and so symbolizes the presence of God's Holy Spirit at Pentecost.  Red is considered the color of the Church, since red can also symbolize the blood of martyrs.  Red represents sacrifice because it signifies the shed blood of Christ's disciples who gave their lives as a witness to the glory of our risen Lord.  It is indeed proper and fitting for us to mark this day (and this week) with the holy color of red.