[thaumasiotes]

A couple points:

- That map is factually incorrect. Sunday is considered the end of the week in China, not the beginning. I note in passing that the image file is attributed to "own work" -- we all make mistakes.

- The 天 of 星期天 doesn't have the sense of "sky"　or "heaven". It has the sense (well, originally had the sense) of "day", the unit of time. Compare 天天 "every day"; 今天 "today"; 明天 "tomorrow". We know that this is the original sense because the name descends from the Christian term 礼拜天 "the day of worship". Consider also that 周天 alternates with 周日, and while 日 shares the sense "day" it does not share the sense "sky".