Where does “четверг” come from?
четверг (Russian) comes from Old East Slavic четвьртъкъ, from Proto-Slavic četvьrtъkъ, from Proto-Slavic četvьrtъ, from Proto-Balto-Slavic ketwirtas, from Proto-Indo-European *kʷetwr̥tós.
четверг (Russian): Thursday
Ancestry of “четверг”, step by step