"We have no way of knowing if today is Tuesday. We just have to trust someone has been keeping count since the first one". When was this first one? Have we been keeping count ever since, or have we ever lost track?

by Dctreu

This was, I believe, a post on r/showerthoughts recently, but it got me thinking: when did we define days of the week, and if we were to go back in time one day at a time, would there be a point at which the day would not be just the previous day? Did we ever skip or maybe even lose track completely, perhaps different parts of the world may have had the same weekday on different days at one point?

Sorry if this questions is a bit vague, but in general I'm asking how the days of the week appeared, and how they've been kept track of since. Is there actually now a real definition?

Holy_Shit_HeckHounds

How confident are we that the year is actually and exactly 2016? Is it possible that at some point in the last 2000 years there were any significant timekeeping mistakes? by /u/sunagainstgold

When was it decided which day of the week it was, in order for today to be Wednesday? by /u/yodatsracist

Indeed the whole FAQ section on Calendars and the following one on Clocks may be of interest (both of the linked threads are taken from those sections).