I was studying the names of the planets and noticed some similarities. Are the days of the week named after the planets?
Yes, they are, and it comes from Western Influence.
日曜, 月曜, 火曜, 水曜, 木曜, 金曜, 土曜 are Classical Chinese names for the Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, respectively.
English names for the days of the week are mostly Germanic names for the same planets. Sunday, Monday, and Saturday are obvious, but we have
- Tuesday (Norse: Tiw for Mars)
- Wednesday (Norse: Odin; Old English: Wōden for Mercury)
- Thursday (Norse: Thor for Jupiter)
- Friday (Norse: Frigg for Venus)
Chinese no longer uses this name scheme, but Korean and Japanese still use this system.