EDIT: got 'monday' and 'sunday' the wrong way round
EDIT: got 'monday' and 'sunday' the wrong way round
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.
In other Slavic languages it's mostly some form of ty + djen' (Polish tydzien Croatian tjedan Czech týden Belarusian тыдзень Ukrainian тиждень). But see Bulgarian (седмица seven-thing (fem.)).
I suspect (without evidence) that, in Russian, it's referring to Sundays as a way of reckoning weeks.
week:Sunday :: year:summer :: month::moon. Cf. "When I was but 10 summers old", "Many moons ago", etc.
That would fit with the use of the archaic word for Sunday as well.