How important is Sunday? According to the Catechism of the Catholic Church and the Youth Catechism of the Catholic Church (YOUCAT) Sunday is an extremely important day. It is the most important day of the week.
Sunday, the “Lord’s Day,” is the principal day for the celebration of the Eucharist because it is the day of the Resurrection. It is the pre-eminent day of the liturgical assembly, the day of the Christian family, and the day of joy and rest from work. Sunday is “the foundation and kernel of the whole liturgical year” (SC 106).   – Catechism of the Catholic Church #1193
Sunday is the center of Christian time, for on Sunday we celebrate Christ’s Resurrection, and every Sunday is a miniature Easter. If Sunday is disregarded or abolished, only workdays are left in the week. Man, who was created for joy, degenerates into a workhorse and a mindless consumer. We must learn on earth how to celebrate properly, or else we will not know what to do in heaven. Heaven is an endless Sunday.   – Youth Catechism of the Catholic Church #187