The thought started when I was thinking about 'The Sabbath' being Saturday/Sunday and that the days of the week aren't exactly tied to anything other than their historical context... Looking back in history do we have an uninterrupted view from when a seven day week started until now to say that today is definitely a Sunday, or is it possible that we gained/lost some days somewhere and that today should actually be a Wednesday or a Friday if we had been keeping better track... Would things like Leap Days or the restructuring of the calendar have any effect on it either?
I can't think of a way that a day would be lost anywhere, except when the calendar was realigned, and the last time this was done that I know much about was in Britain in 1752.
On that occasion, the date was advanced by 11 days, so Wednesday September 2nd 1752 was followed by Thursday September 14th. So although the date changed, the days of the week of the week stayed in order.