Not-Doing can be a false alias: the week is nedelya and ponedelnik may thus mean "one going with the week*, i.e. starting it.
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.
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.