From the time of the Mosaic law, the People of God have observed fixed feasts, beginning with Passover, to commemorate the astonishing actions of the Savior God, to give him thanks for them, to perpetuate their remembrance, and to teach new generations to conform their conduct to them. In the age of the Church, between the Passover of Christ already accomplished once for all, and its consummation in the kingdom of God, the liturgy celebrated on fixed days bears the imprint of the newness of the mystery of Christ.

Source: Cathecism of the Catholic Church

Corpus Christi – Celebrated on the Thursday after Trinity Sunday, 60 days after Easter, or, in countries where it is not a holy day of obligation, on the following Sunday.