For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it. Ex. 20:11
"Dean Stanley (Church of England) writes:
the weekly Christian festival, is, in a great measure, owing to the union of pagan and Christian sentiment with which the first day of the week was recommended by Constantine to his subjects, pagan and Christian alike, as the 'venerable day of the sun.'"—"History of the Eastern Church," lecture 6, par. 15." W.A. Spicer