Acts 20:7 states: And upon the first day of the week (Sunday), when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
The Didache, is also known as “The Teaching of the Twelve Apostles”. It is an ancient Christian text that was probably a catechism, used by the early Church.
And on the Lord’s own day gather yourselves together and break bread and give thanks, first confessing your transgressions, that your sacrifice may be pure. And let no man, having his dispute with his fellow, join your assembly until they have been reconciled, that your sacrifice may not be defiled; for this sacrifice it is that was spoken of by the Lord; Didache, paragraph 14 (probably written between 70-140 A.D.)