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!
EDIT: so question is not about the origin of 7-day week and sequence of weekday names, but about the exact reference point (day) of today’s weekday countdown. From when have people stopped adding or ommiting any adjustment ‘out-of-week’ days (like in Babylon or Rome) and kept counting to seven till today? In other words, there should be a point exactly N x 7 days ago from which the 7-day countdown has not been interrupted. Or at least the earliest known day in history that everyone on Earth agreed upon as a reference point
EDIT 2: Solved by https://lemmy.world/comment/1852458 Thanks everyone!
You have to go back on a per-calendar basis. The Chinese calendar will have a different answer to this question than the European calendar, for example. It is likely that different calendar systems came up with continuous 7-day cycles at different times and in different cultures without referring to each other, because the 7-day cycle maps to their shared observations of the moon cycles.