106 By a tradition handed down from the apostles which took its origin from the very day of Christ's resurrection, the Church celebrates the paschal mystery every eighth day; with good reason this, then, bears the name of the Lord's day or Sunday. For on this day Christ's faithful are bound to come together into one place so that; by hearing the word of God and taking part in the eucharist, they may call to mind the passion, the resurrection and the glorification of the Lord Jesus, and may thank God who "has begotten them again, through the resurrection of Jesus Christ from the dead, unto a living hope" (1 Pet. 1:3). Hence the Lord's day is the original feast day, and it should be proposed to the piety of the faithful and taught to them so that it may become in fact a day of joy and of freedom from work. Other celebrations, unless they be truly of greatest importance, shall not have precedence over the Sunday which is the foundation and kernel of the whole liturgical year.
Source: Sacrosanctum Concilium (Vatican.va)