Older thread (started by my humble self) raising this very question: Are weekdays a sequence uninterrupted for millennia?
I’d also like to add that it’s a bit oversimplifying to say that the Gregorian calendar was adopted by most of the Western world in 1582. It was established by a papal bull and, accordingly, obeyed only by the Catholic countries initially - which meant a lot in these post-Reformation days. Protestants (England and its dependencies, Scandinavia, the Low Countries, much of Germany) ignored it, and so did Orthodox Christianity (which I think can be seen as part of the Western world). The absence of England, in particular, is the cause for one of my favourite quirks in all of human history, namely the fact that Shakespeare and Cervantes died on the same date (23 April 1616) yet ten days apart.