The names of the days of the week in Japanese are based on the 5 elements plus sun and moon. Moonday, Fireday, Waterday, Woodday, Metalday, Earthday, Sunday.
Incidentally, those same elements show up in European languages too, including English. Sunday and Monday are obvious, but the rest of the English days of the week are named after Norse gods who are equivalent to Roman gods who are aligned with the planets which are named after the elements in Japanese. Well, you could also think of the astrological theme as primary, since sun and moon are thrown in.