The celebration of Trinity Sunday began among Western Christians in the 10th century and developed slowly until it was formally established on the Sunday after Pentecost by Pope John XXII (1316-1334).
On the Day of Pentecost we celebrate the gift of the Holy Spirit descending in a mighty rush of wind and flame to inspire the church’s proclamation of Christ’s rising and to empower its mission and ministry to the world. (See Acts 2:1-13; see also Joel 2:28-32.)
The notion of Easter as a season of 50 days ending at Pentecost is patterned after the ancient Jewish festival of seven weeks that extended from the beginning of the barley harvest (on the second day after the beginning of Passover) to the end of the wheat harvest at the Festival of Weeks or Shavuot (see Deuteronomy 16:9-12). The Festival of Weeks later came to be called Pentecost (“50th day”) by Greek speaking Jews. In Jewish tradition, Shavuot also marks the giving of the law to Moses at Sinai; this liturgical link may inform Paul’s discussions of the law and the Spirit (see Romans 8, 2 Corinthians 3 and Galatians 3).