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

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.