The Christian community, coming out of Judaism, modeled many of its celebrations on the Jewish calendar. Pentecost, as its name implies, marks the 50th day after Easter as Shavuot marks the 50th day after Passover.
For the church Pentecost is celebrated as the outpouring of God's Holy Spirit on the community. It is, in a sense, the "birthday" of the church.
In many churches the story of Pentecost is read from the Acts of the Apostles, which describes this gift of the Spirit as a moment that unites all peoples. While the early followers of Jesus were from different places around the Mediterranean and spoke different languages and had different cultures, the Spirit made them one people.
In many Christian traditions red is the color of the day as it signifies the "fire" and the "power" of the Spirit in our lives. This feast brings to a close the Easter season. The remainder of the year is Ordinary time or Time after Pentecost until Advent, which begins again the cycle of the Christian calendar.