Wikipedia (08 Mar 2013, 08:36)
Emperor Constantine I decrees that the dies Solis Invicti (sun-day) is the day of rest in the Empire.
Iin 321, Constantine instructed that Christians and non-Christians should be united in observing the venerable day of the sun, referencing the sun-worship that Aurelian had established as an official cult. Furthermore, and long after his oft alleged "conversion" to Christianity, Constantine's coinage continued to carry the symbols of the sun. Even after the pagan gods had disappeared from the coinage, Christian symbols appeared only as Constantine's personal attributes: the chi rho between his hands or on his labarum, but never on the coin itself. Even when Constantine dedicated the new capital of Constantinople, which became the seat of Byzantine Christianity for a millennium, he did so wearing the Apollonian sun-rayed Diadem; no Christian symbols were present at this dedication.