Unlike the day, year, or lunar month, there’s no natural unit of time corresponding to the week. So it’s hardly surprising that different societies have chosen different lengths. Ten is one obvious choice (there’s a reason we refer to number places as digits, after all). But aside from the obvious Biblical origins, why seven?

Well, seven days roughly corresponds to a phase of the moon. But humans have long had a fascination with the number seven, no doubt influenced by the seven heavenly bodies: the sun, the moon, and the five visible planets. Sunday, Monday (moon day) and Saturday (Saturn day) are obvious in English, but Tuesday through Friday are a little less clear: you have to work out which Norse god the name comes from—Tyr, Wotan, Thor, Frigg—and convert to the corresponding Roman god—Mars, Mercury, Jupiter, Venus. It’s much clearer in Romance languages, as I discovered when I studied Spanish a few years ago. Wikipedia also has a nice table of weekday names in various languages.

On a related note, if February were a full month, today would be February 30. It turns out there’ve been a few of those in relatively modern times, including an extra-long leap year in Sweden in 1712, and two in 1930-1931, when the Soviet Union tried to use a “revolutionary calendar.” (Funny how those never seem to catch on.)