this post was submitted on 29 Jul 2023
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7 days is about how long a lunar "phase" is. A full lunar cycle is 28 days and there are 4 phases. Counting 7-day cycles is older than history. Counting moons (28-day cycles) is older than history.
Here is the problem, because actual lunar cycle is 29.5 days long, so if we simply count its phases with whole 7 days it will quickly run out of sync. Therefore Babylonians and other ancient folks added a couple of 'out-of-week' days every now an then to compensate the difference.
The moon has been around a long long time. Everyone figured that out long before recorded history. Every culture dealt with it differently. Each phase was 7.4 days long. So each phase was countable as a 7 day week but an adjustment day is needed somewhere. It could be every 2 weeks, it could be end of the month, whatever. But the 7 days comes from the amount of time it takes to go from one visible lunar phase to another.
I'm not arguing with that, but my question is different: where in history is the exact reference point (day) of today's weekday countdown? From when have people decided to stop adding or subtracting adjustment days and kept counting till today? The might have been some shifts along the way, but 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.
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.