Earlier quoted context omitted.
I thought that that in Slavic languages Sunday would be the first day of the week, since the Polish for 'monday' means 'after sunday'. EDIT: got 'monday' and 'sunday' the wrong way round
In Russian, Sunday is "Resurrection Day" and Monday is "The Thing concerning Not-Doing being in the Past-Perfective Tense" (po + ne+del + nik) It looks like Polish is similar except that Sunday is "Not-Doing"
Altough I'm not sure since the sibling proto-slavic explanation makes much sense. Fun fact: slavic languages split off in medieval times when the calendar and the week were already thorougly taken care of.