The Lord's day
Sunday is the pre-eminent day for the liturgical assembly, when the faithful gather "to listen to the word of God and take part in the Eucharist, thus calling to mind the Passion, Resurrection, and glory of the Lord Jesus, and giving thanks to God who 'has begotten them again, by the resurrection of Jesus Christ from the dead' unto a living hope":
When we ponder, O Christ, the marvels accomplished on this day, the Sunday of your holy resurrection, we say: "Blessed is Sunday, for on it began creation . . . The world's salvation ... The renewal of the human race .... On Sunday heaven and earth rejoiced and the whole universe was filled with light. Blessed is Sunday, for on it were opened the gates of paradise so that Adam and all the exiles might enter it without fear.
Source: Catechism of the Catholic Church (Vatican.va)