[torstenvl]

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"

[xyzzyz]

In Polish, Sunday is „niedziela”, and Monday is also „poniedziałek”, so it actually makes more sense than in Russian :) I suspect “niedziela” («неделя») was the original, proto-Slavic word for the day of the week, as some variations of it are used for I think all Slavic languages except Russian, who at some point decided to rename it to celebrate Resurrection.

[thriftwy]

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.

[torstenvl]

Only in Russian.

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.

[LudwigNagasena]

Ponedelnik etymologically means the day after the not-doing day. Nowadays the connection is lost because Sunday is no longer called “nedelja”.

[jq-r]

Depends ;). In Croatia its still called nedjelja (Sunday) and ponedjeljak (Monday).

[dullcrisp]

Hmm, and the week is the not-done-thing?

[ivlad]

Etymology is hard, the week “ne-delya” could be understood as something that cannot be divided, “not-divisible”. I am not sure which one is correct.