Six days might be the most they could force people to work before giving them a day off? (The French tried 10 day weeks and that was too long between rests for most people) When we became more civilized we let the wage slaves take two days off. It\’s coincidentally the closest number of days to 1/4 lunar cycle and the length is a capitalist\’s wet dream (a third of an extra payment in a average month is little enough to make people who don\’t like math not care but could be almost the entire profit margin, an oppressed peon who doesn\’t have enough to pay the whole month up front can now be oppressed even more while the price still superficially looks fair. I still meet people that don\’t realize how big an extra third of a week a month is, then I tell them that there are 52 and 1/7th and change weeks in a year and only 48 quarter months and they\’re like oh sht, I never thought of that before. )
What? It may have started out as the closest integer to the length of a quarter lunation but I suspect that it being the absolute perfect length to squeeze as much as possible out of worker (ironically, mostly chattel slaves) and buyers made it catch on in Ancient Rome. I\’ve heard that Rome did some experimention on week lengths and found out that the 7 day work rhythm was the best (though maybe the good jobs like architect had two days off just like today and the laborers had one?
I don\’t know). tl;dr: Nature + superstition. A month was originally the time it took for the Moon to go through its phases and come all the way back around to when it started. That length of time – which we now called the synodic month to distinguish it from other kinds of months like the ones on the calendar – is not a whole number of days. (It would be a coincidence if it were, like finding a branch in the woods that happened to be exactly 100 centimeters long). In fact, a synodic month is not even a constant amount of time, since it depends on where the Earth, Moon, and Sun are relative to each other, and the speed of the Earth around the Sun changes throughout the year. But on average, it s about 29б days long. When people were just looking at the sky and talking about how many moons it would be until some event, they didn t need to be very precise. But eventually people needed more precision and started naming individual days; a system for naming days is what we call a calendar. The month was a logical way to divide days up into easy-to-count chunks.
But a 29б-day month is not convenient for a calendar; a calendar where the date changes in the middle of the day one day out of the month would make things more complicated. So the calendars generally had a 30-day month followed by a 29-day month followed by a 30-day month, and so on. That s the first reason months have different numbers of days. The Romans thought even numbers were unlucky, so they didn t use 30-day months; instead of the pattern (30, 29, 30, 29, 30, 29) their months went (31, 29, 29, 31, 29, 29). That s where we get the idea of 31-day months, even though the Moon never takes that long to go through its phases. When using the calendar to predict things that depend on the seasons (like harvest time), you need to know how long a year is. And a year is not a whole number of months (again, that would be a big coincidence). The moon goes through its phases twelve times in a year, but twelve 29б-day months make only 354 days – there s 11 more days before you get to the same spot in the seasons. Most calendars deal with this by inserting extra months – leap months – every few years. But since you need to be able to do math to tell when to do that, and most people couldn t do math, in Rome the people who controlled the calendar took advantage, and things got very confusing.
Some other calendars – like the Egyptian one – went another way: they decided to throw away the whole idea that months had to match the Moon. Julius Caesar decided this was a good idea and, with the help of some Egyptian astronomers, changed the Roman calendar to do the same thing. To get the extra 11 days needed for a full year, they added days to most of the 29-day months so they had 30 or 31 days, not worrying about the even-number superstition. But superstition still played a role, because February was thought to be an unlucky month, so it was left at 29 days, and even then only in leap years; it got only 28 days in regular years. Putting the leap day there was also done partly for that reason – changing lengths was just creepy, so better make it the month that s already unlucky. (The other reason was tradition: March had once been the start of the year, and putting the extra day at the end of the year made sense. But March wasn t the first month of the new calendar – January was, just like it is today. )