"Do this in memory of me"
It was above all on "the first day of the week," Sunday, the day of Jesus' resurrection, that the Christians met "to break bread." From that time on down to our own day the celebration of the Eucharist has been continued so that today we encounter it everywhere in the Church with the same fundamental structure. It remains the center of the Church's life.
Source: Catechism of the Catholic Church (Vatican.va)