Where in history is the starting point of modern week day count? - Lemmy.world

I mean, if today i.e. is Sunday then someone long time ago should have said “Today will be Sunday” for the first time in a period from today that is multiple of seven. I was assuming that it was Pope Gregory XIII in October 1582, but looks like he is not. I failed in googling and duckduckgoing out the answer, so I ask for Lemmy’s collective wisdom!