Ever wondered why a week contains 7 days ? Seven is actually a weid odd number. Ten or six days would have been much easier : a year is 365 days, so nearly 60 'weeks' of six days, and a month would have been exactly 5 weeks. It seems that the 7 day week is a remnant of the Babylonians, who were vivid astronomers : their calendar was based on the moon, from which the month originates. But they needed something smaller than a month, and larger than a day. Four weeks of seven days were closest to the 29.5 day moon cycle, so they stick with it.

Names of weekdays is also a fun subject : they were named after the planets, which in their turn were named after the gods. In Roman languages, weekday names contain the Roman gods (lundi, mardi, ...), whereas in German based languages, the days contain the names of the Scandinavian gods (Wodan, Freya, ...)