Article I, Section 7 of the United States Constitution says that the President has 10 days to veto a bill. If he doesn't veto it within those ten days, "Sunday excepted," then the bill becomes law.
Here is the absolute refutation that the framers wanted to remove religion completely from government in the unamended Constitution. If they had wanted to do so, they certainly wouldn't have put that phrase in there, effectively giving the President ten days to consider the bill, with Sunday as a day of rest.
The question has to be asked, "Why did they pick Sunday?" If they really wanted to make it clear that they wanted to keep religion completely out of the government, could have chosen Saturday, or any other day. But why Sunday?
The answer goes back several centuries, when Constantine declared Christianity the official religion of the Roman Empire. He declared Sunday to be an official day of rest, retaining the original name, presumably, to please his still heathen subjects. Sunday was picked because that is the day that Christ rose from the dead.
From that time until this, Sunday has been considered a sacred day throughout the history of Western Civilization. One hardly needs to read a history book to understand this. From the perspective of Christianity, Christians for several centuries began referring to Sunday as "the Sabbath," including such famous preachers as John Wesley and Charles Haddon Spurgeon. While this is technically not accurate, it is safe to say that history teaches us that Sunday is the Christian Sabbath, while Saturday is the Jewish Sabbath (the latter actually beginning on Friday night).
The framers of our Constitution specifically wrote a recognition of that sacred day into the Constitution. The phrase literally allows the President to relinquish Sunday as a day of consideration of a bill before he must sign it. He has ten days to sign the bill, with the exception of Sunday, or it becomes law.
Note that the framers specifically mention Sunday. They do not mention ten days, with an arbitrary day of rest, but rather Sunday specifically. There is absolutely no reason to divorce ourselves from the natural understanding that they picked this day due to its importance in the history of Western Civilization, which we have briefly seen.
So a question, arguably rhetorical, must be asked: If the framers wanted
to divorce religion completely from government, why did they specifically
make provision for Sunday as a day of rest within the Constitution?
 For information regarding the history of Sunday, see Daniel J. Boorstin's
The Discoverers, pp. 15, 16. See also Europe: A History by
Norman Davies, pp. 208, 209.