How did the Sabbath day (Saturday) when we worship together turn into Sunday? Can you tell me where it is in the catechism's doctrine? I know a little about the Council of Trent but cannot find where this issue is addressed.
The change in the day of worship came with the changes in the covenant. God promised the Israelites, "Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah - not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the LORD" (Jeremiah 31:31-32).
The book of Hebrews, especially from chapters 3 to 10, speaks in detail about many of these changes. The Old Law required daily sacrifices, the Law of Christ is based on one perfect sacrifice. The Old Law set aside a group of people, the Levites, to act as mediators between man and God, the Law of Christ states that all Christians serve as priests before God. The Old Law accused man of sin but could only offer a future hope of salvation, the Law of Christ offers salvation based on the shed blood of Jesus. Among the many changes was a change in the day of worship. Offerings are to be taken up on the first day of the week. "On the first day of the week let each one of you lay something aside, storing up as he may prosper ..." (I Corinthians 16:2). The example of early Christians is that they partook of the Lord's Supper on the first day of the week. "Now on the first day of the week, when the disciples came together to break bread, ..." (Acts 20:7).
You claim to have heard that it changed at the council of Trent, which took place between 1545 and 1563 A.D. This is an unusually late date; most Sabbatarians claim the change took place under Constantine. However, it is trivial to show that Christians worshiped on Sunday long before Constantine. See "You only worship on Sunday because Constantine changed the day of worship" for the evidence.