What makes Sunday so special? I presume it means something other than the translation given ("boss Japan").
The original name of Sunday is Chúa nhật (the day of God (Chúa is God, nhật is a Sino-Vietnamese word which means "day")). This name was created by Western missionaries when they came to Vietnam and was used commonly by Vietnamese Christians. But most of Vietnamese people are non-religious, not Christians. So, to avoid showing the involvement to Christianity, they changed Chúa (God) into Chủ (Master) because the word Chúa also has a meaning of being the boss of someone like Chủ. That was the reason why we have Chủ nhật (the day of Master) although we don't know who this Master is.
Right, that is very interesting. I figured the Western calendar was brought over by the French.