Christians celebrate Pentecost to commemorate Jesus' early followers receiving the Holy Spirit as described in the second chapter of the biblical book of Acts. The first Pentecost occurred a few weeks after Jesus' resurrection.
The word "Pentecost" comes from the Greek word "pentekostos," meaning "50." The Hebrews created the word from a Jewish celebration described in Leviticus 23:16, which instructs the Jews to put 50 days or seven weeks between the end of Passover and the next holiday.
Each Christian church celebrates Pentecost differently. Liturgical churches use the color red to symbolize the power and fire of the Holy Spirit. Other churches accept new members on Pentecost to commemorate new people joining Jesus' church after Peter's sermon.