Fifty days after the Resurrection, the Holy Spirit descends upon the apostles in the form of tongues of fire, and the frightened disciples become fearless preachers, speaking so that every nation hears the Gospel in its own tongue. Pentecost is the birthday of the Church: what Christ accomplished is now poured out on all who believe.
With Pentecost the revelation of the Holy Trinity is complete. At Theophany the Trinity was shown at the Jordan; now the Spirit, sent by the Father through the Son, comes to dwell in the Church and in each of the baptized. This is why the feast is also kept as Trinity Sunday: the whole mystery of the one God in three Persons stands revealed.
The confusion of Babel, where pride scattered the nations into many tongues, is here undone: the Spirit gathers the nations into one faith. On this day, for the first time since Pascha, the Church kneels again, in the great kneeling prayers of Pentecost, asking the Spirit to abide. The feast is measured from Pascha and always falls on a Sunday, fifty days after the Resurrection.