Chinese uses the numbers for days of the week, so 星期一,星期二,星期三,星期四,星期五,星期六,星期日 for Monday to Sunday. They can use 周 or 礼拜 instead of 星期.

I remembered Japanese copied the Chinese at something there, but, whoops, it was the names of the planets that were copied. So the Chinese names of Mars(火星), Mercury(水星), Jupiter(木星), Venus(金星), and Saturn(土星) are copied onto the same days as planets that Romance languages name their days after.

FYI, 日 is not "seven". It's "sun" or "day". It's almost like someone had a sense of humor when they named that day.

My interpretation has always been that 日 is meant to represent "zero", but I never put any thought into why they didn't just call it 星期零.

"星期日" is generally written Chinese, but in speech Chinese say "星期天" where 天 could mean "God" when the 7-day week was copied from "Christian" Europe. I just found http://www.cjvlang.com/Dow/dowchin.html to explain the history of Chinese and Japanese day names in detail.

I've noticed while living in China that, whereas Westerners like me virtually always remember what day of week it is but often forget the date in the month, Chinese are more likely to remember the day of the month and forget the day of the week.

Ah. I grew up in a Cantonese-speaking Christian family in America, so I've never heard 星期天 used in speech at all.

Confusingly, the "seven luminaries" system in that article aren't just referencing the planets: they're also the names of Sun, Moon, Fire, Water, Wood, Metal, and Earth. I did get that sorted out by looking up Western alchemical ordering for the planets, which is in the same order. Wikipedia helpfully provides a table that includes the old names for the planets as well:

http://en.wikipedia.org/wiki/Classical_planet#East_Asia