I thought of this question after reading about day-wise restrictions on shaving/haircut
These weekly day-wise restrictions must have started after a 7 day week was established.
Which is the oldest known reference to a 7 day week (or a particular day of the week) in Hindu scriptures?
(Note - This question lists some (not all) scriptures with references to
the week or
weekdays, but it does it mention which one is the oldest)