The Day of Pentecost celebrates the birth of the Christian Church – it marks the day when Jesus' disciples felt the Holy Spirit come upon them and empower them to tell Jesus' story to the world. Red, the color associated with the Holy Spirit, is the color for the Day of Pentecost.
The season after Pentecost is the sixth and final season of the church year. This season, sometimes referred to as "ordinary time," is the longest of the Church year, lasting through summer and well into late autumn, ending with the coming of Advent. During the season after Pentecost we recount the stories of Jesus' ministry, read and study passages from the Old Testament, and discuss letters written by disciples for members of the early church. The color for the Season after Pentecost is green, appropriate for the season of growth.
And after this "final' annual season we return to Advent and have the opportunity to deepen our knowledge of and commitment to Christ and God.