Why Sunday?

Basil Lourié directs us to a Late Antique controversy over what's the most important day of the week.

First is "The Epistle on Sunday", delivered from Christ Himself to the author. This seems to be about the most-canonical Scripture that Christians these days don't read anymore, extant in hundreds of MSS. Some say it dates to AD 540ish in Jerusalem, although Lourié would nuance that. A rival tradition argues, from a Hebraic tradition, that the week should start on Wednesday because by Genesis One no sun existed to mark time before that; a "day" before that Wednesday has no calendric meaning.

The debate hit Christendom during the AD 450s in the wake of Chalcedon, as did many controversies. Theodosius of Jerusalem refused this Council.

If Wednesday was just as good as Sunday, other days would get their place in weekly devotions - which, for Christians, meant Friday. Friday became very important for Near Eastern Christendom, not just for Lent. Lourié thinks that's whence the Arabs got the idea. And then the Muslims.