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I would like to know if there is detailed information as to why days of the week use on-yomi readings, for example
月 in 月曜日 【げつようび】
Were these readings (as spoken) imported from the Chinese? And if so, is it known what days of the week were called prior to Chinese influence?
As for why they use on-yomi, it's just because they were lifted right out of Chinese by monks back in the Heian period. That's the easy part of this question, but interestingly it's really hard for me to find anything about what the days of the week were called before the introduction of 七曜. Apparently the naming of the days of the week based on the 7 visible planets came through an old Buddhist text, quite a mouthful, called 文殊師利菩薩及諸仙所説吉凶時日善悪宿曜経{もんじゅしりぼさつきゅうしょせんしょせつきっきょうじじつぜんあくすくようきょう} (or just 宿曜経 for short), with the 7 day system going as far back as ancient Babylonia.
Before this, however, I can't find anything too comprehensive. There is a system of days that also originated in China called the 六曜{ろくよう}, a system of a 6 day week where each month is divided up roughly into 5 segments. These 6 days are 先勝, 友引, 先負, 仏滅, 大安, 赤口, and they represent certain astrology-tyle 'lucky' qualities. The first is "lucky in the morning but not the afternoon." The next is good for lawsuits ad business. The third is unlucky in the morning. The fourth a very unlucky day (Buddha's death). The fifth is lucky, and the 6th is unlucky except at lunch time. According to wikipedia, though, this was actually introduced after the current system. So I guess the search continues.
There is another very old system of measuring time known as the 旬{じゅん}, but this also unfortunately does not refer to specific days. Rather it refers to groupings of 10 days, so in a month you have 上旬・中旬・下旬 to refer to the first, middle, and final 10 days of the month. This system of measuring time goes way back to the oldest writings in China, but you're not going to find your days of the week here.
With the above I guess it is possible that they used 十干{じっかん} to name days, but I'm not sure. The 10 are 甲・乙・丙・丁・戊・己・庚・辛・壬・癸, and they each apparently match up with one of the Chinese elements that were adopted into the names of days (look at the table on Wikipedia). Like I said, though, whether or not this naming convention was used in Japan I am not sure.
None of these is a really satisfactory answer to the question, but in all the searching I did I could not find any explicit information about the names of the days of the week in Japanese before this. However I did learn a lot about it otherwise so I wanted to write it out here anyway.
For further reading check out these sites:
http://www.cjvlang.com/Dow/dowjpn.html (English) http://ja.wikipedia.org/wiki/%E4%BA%8C%E5%8D%81%E5%85%AB%E5%AE%BF (another astrological grouping of 28 days from China) http://koyomi.vis.ne.jp/doc/mlwa/200611070.htm http://ja.wikipedia.org/wiki/%E4%B8%83%E6%9B%9C http://ja.wikipedia.org/wiki/%E6%9B%9C%E6%97%A5
