Keski-viikko is indeed Mid-week.

Interesting, given that I found, whil(e,st) searching for the Finnish day names, the datum that German Mittwoch was a Christian reaction to Mercury-day being named after a pagan god. But why just Woden's day and not the others?

The only Western Gemanic language that I know that lost the -g at the end of day is English but one would expect Finnish to have gotten their loans from a Northern Germanic language. How is it in Scandanavian languages?