Sure, Monday through Saturday are numbered 1 to 6, but Sunday is "weekday Sky" or "weekday Heaven", which is considered to come before "weekday one".[1]
Portuguese uses ordinal numbers to name days from Monday to Friday, starting from Sunday: Monday is segunda-feria (second day), Tuesday is terça-feria (third day), all the way to Friday, or sexta-feria (sixth day).
Interestingly, there's no first day: Saturday and Sunday are sábado and domingo, respectively.
- 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".
Interestingly, there's no first day: Saturday and Sunday are sábado and domingo, respectively.