The New Testament records Christians gathering on the first day of the week (Acts 20:7; 1 Corinthians 16:2), and Revelation speaks of the Lord’s Day (Revelation 1:10). The Didache commands believers to gather on the Lord’s Day for the breaking of bread (Didache 14). Ignatius contrasts living according to the Lord’s Day with an older manner of observance, and Justin Martyr gives a detailed account of Sunday assembly in the mid-second century (First Apology 67).
Constantine’s fourth-century legislation gave Sunday a civil status in the empire. It did not travel backward in time to create worship already documented by Christian sources. The early record includes complexity—Jewish Christians and others could retain Sabbath practices—but the basic claim that Constantine invented Sunday Christian assembly is untenable.
Primary sources
Didache 14: https://www.newadvent.org/fathers/0714.htm
Justin, First Apology 67: https://www.newadvent.org/fathers/0126.htm