Christianity Today magazine has an interesting article about Craig Harline's new book called "Sunday: A History of the First Day from Babylonia to the Superbowl". The article contains an excerpt from the book, which shows how so-called Christians adopted the pagan 'Sun Day' into their worship. Here is a sample:
And when Christianity moved into northern Europe, "Sunday" was so common among Christians that the new northern converts simply used that name exclusively. Just as pagan forms had been conquered and appropriated, now the greatest pagan day was absorbed as well. Emerging vernacular languages around the mediterranean, where the early church was born and raised, certainly retained "Lord's Day" in common usage, but a good Christian in either north or south could now find as much Christian imagery in "Sunday" as they pleased, and thus uttered the word without a second thought. Only the later vernacular languages of eastern Christianity, and Portuguese in the west, would reject all planetary names for the days of the week as offensively pagan.