

The number of days in a week wasn’t chosen as an arbitrary number, it was chosen because it’s the length of 1 moon phase. 4 phases of the moon is 1 cycle (new moon to new moon), or 1 month (28 days).
The fact that we now have most months with 30 or 31 days is basically a hack to try to line up 12 months to 1 year (1 orbital period of the earth), but that was never going to work!








The Roman calendar originally didn’t last all year round. It began in the spring and ended at the start of winter, and then the winter period just didn’t exist on the calendar.
Calendars were invented to plan the growing of crops, so tracking the days in winter didn’t have any purpose, so they left winter out.