I always understood the first day of the week to be Monday, in Portuguese it is Sunday - Domingo, with Monday being Segunda Feira (second, feira has no meaning in its own right I believe).
When you learn a second language as an adult you learn even more about your native language and sometimes you realise that what seems simple and automatic makes no sense, eg, phrasal verbs, eg, put up with, fed up, etc.
I don't know about anyone else but I found English grammar mechanics difficult to understand at school but when I started a 2nd language it got me to think more about my language.
Anyway, that is the limit of my knowledge on this one.
