If you’re referring to Slavic languages, they’re counted from Sunday.

• Sunday: “no work”

• Monday: “[first] after Sunday”

• Tuesday: “second [after Sunday]”

• Wednesday: “middle”

• Thursday: “fourth [after Sunday]”

• Friday: “fifth [after Sunday]”

• Saturday: “sabbath”

Treating the work week as the start of the calendar week is a 20th century invention, as is the five‐day work week. Historically, Slavic calendars began on Sunday: https://www.russianhistorymuseum.org/wp-content/uploads/2025...