Did Constantine Make Sunday a Legally Protected Day of Rest?

321 CE · Emperor: Constantine · Evidence: Roman legislation (Codex Justinianus 3.12.2)

What the ancient record says

Yes, with careful wording. In 321 CE Constantine issued legislation protecting rest on the day associated with the Sun, the famous wording refers to the "venerable day of the Sun" (dies Solis); the law ordered judges and city dwellers to rest and workshops to close, with an agricultural exception (farming delay could be costly).

Did Constantine invent Sunday observance? No, it existed before him.

What was new: he brought imperial civil law into the picture, the difference between a community voluntarily gathering on a day and an emperor making it a legally protected civil rest day. The wording does not say "the Christian Sabbath", do not rewrite it into later religious terminology.

Not everyone had to stop all work (agricultural exception). Accurate: Constantine established a significant Sunday-rest law affecting courts, urban populations, and workshops, permitting agricultural exceptions.

Did it by itself "change Shabbath to Sunday"? That collapses centuries into one decree, what it demonstrably does is give Sunday civil legal status; later ecclesiastical rules (Laodicea Canon 29) contrast Sunday honor with working on Shabbath; the cumulative development matters more than a slogan.