Also, the week itself as a 'hard' delineator of time also appears to be fairly recent:

> Henkin treats the seven-day week as a social and cultural institution, focusing primarily on the nineteenth-century United States. Unlike years, months, and days, which have astronomical foundations, the week is an entirely historical and social construct with no scientific basis. The study argues that the early American republic was an unusually week-oriented society, in part because American colonists abandoned most of the annual feast days common in Europe, leaving the weekly Sabbath as the primary calendrical anchor. The influence of Puritan settlers, who rejected the Church of England's abundance of saints' days as redolent of Catholicism, contributed to this dominical orientation. Through industrialization, urbanization, and mass education during the nineteenth century, weekly consciousness became increasingly entrenched in American life.

> He identifies several factors that strengthened Americans' associations between specific weekdays and particular activities. Weekly wage payments, which became more common during the antebellum period, tightened the relationship between money and time; workers paid on Saturdays often spent much of their earnings quickly, creating what one passage describes as a financial hourglass pattern across the week. Standardized housework routines assigned specific tasks to particular days, with Monday widely recognized as washing day. Common schools organized curricula around weekly schedules […]

* https://en.wikipedia.org/wiki/The_Week:_A_History_of_the_Unn...

While Christianity's seven day period would make you think it was 'obvious', it tended to be broken up in more Catholic cultures (i.e., not Protestant, perhaps more Puritan), and so was a bit of a 'softer' delineation:

* https://en.wikipedia.org/wiki/Feast_day

In pre-industrial times a large portion of the population was agriculture, and you may go to church, but you still have to take care of animals and such even on the seventh day; it was only in a more 'mechanized culture' (e.g., with factories) that time itself could perhaps be treated more mechanically.

Also also:

* https://en.wikipedia.org/wiki/Seven-day_week

while the seven day week is arbitrary, it is also worth noting that the French Revolution had experimented with a metric ten day week, which ended up being a total bust because laborers only got one week in ten of rest rather than one in seven.