I've always been curious about how exactly people in the Roman empire distinguished Saturday from Sunday, given 7-day weeks (or any kind of week) were not officially recognized or observed.

Perhaps by the time of Constantine, the idea of the week was coming into vogue. Even so, calendar computation must not have been trivial. Knowing what day of the week it was at any given time in cities across the empire would have taken focused work since months were of variable lengths and dates within them were not assigned discrete numbers like ours are. And then there was the whole intercalia problem in midwinter where a block days (the number of which changed sometimes) didn't fall within a month at all.

Just keeping track of Saturday and Sunday, let alone keeping them aligned with calculations in other cities, must have taken a considerable amount of doing not to mention an agreement to allow some central authority to arbitrate inevitable disputes about calculations.

And we think calculating the date of Easter is challenging! Lol