This website describes “Days of the Week in Chinese, Japanese, and Vietnamese”:
There’s something friendly and familiar about the names of the days of the week in English and other Western European languages. Each has its quirks (the Romance languages use Roman gods, the Germanic languages use Germanic gods, Spanish and Italian use ‘Sabbath’ instead of ‘Saturday’) but with a bit of background they fall into an interesting but reassuring pattern.
Not so Chinese, Japanese, and Vietnamese (CJV), which seem completely alien. Chinese and Vietnamese simply count the days of the week; Japanese uses a strange collection of elemental names reminiscent of primitive religion. Given that Chinese and Vietnamese can’t even agree which day to count from, the three languages seem to have little to do with each other, let alone the languages of Europe.
But this appearance is deceptive. A little delving reveals a much more complex picture that is every bit as fascinating as the languages of the West. Ironically, Japanese and Vietnamese turn out to be more faithful to traditional Western concepts of the week than modern English is.