Pentecost Sunday with Christmas and Easter, ranks among the great feasts of Christianity. It commemorates not only the descent of the Holy Spirit upon the Apostles and Disciples, but also the fruits and effects of that event: the completion of the work of redemption, the fullness of grace for the Church and its children, and the gift of faith for all nations.
After Jesus had ascended to heaven from Mt. Olivet, the apostles and disciples returned to the Holy City. They remained together in the Upper Room or Cenacle, the place where Jesus had appeared to them and which may well be called the first Christian church. About a hundred and twenty persons were assembled there. They chose Matthias as an apostle in place of the unhappy Judas; they prayed and waited for the Paraclete.
Ten days had passed, it was Sunday, the seventh Sunday after the resurrection. At about nine o'clock in the morning, as they were together praying fervently, the Holy Spirit descended upon them. Note how all the great theophanies in Christ's life occurred during the course of prayer. After His baptism, for instance, when Jesus was praying the heavens opened and the Holy Spirit descended in the form of a dove; likewise, it was during prayer at night that the transfiguration took place on Tabor.