Pentecost
On that day, the Holy Trinity is fully revealed. Since that day, the Kingdom announced by Christ has been open to those who believe in him: in the humility of the flesh and in faith, they already share in the communion of the Holy Trinity. By his coming, which never ceases, the Holy Spirit causes the world to enter into the "last days," the time of the Church, the Kingdom already inherited though not yet consummated.
We have seen the true Light, we have received the heavenly Spirit, we have found the true faith: we adore the indivisible Trinity, who has saved us.
Source: Catechism of the Catholic Church (Vatican.va)