You are right, originally they did. The answer is catholicism happened:

A religious purist, Jón made it his mission to uproot all remnants of paganism. This included changing the names of the days of the week. Thus Óðinsdagr, "day of Odin", became miðvikudagr, "mid-week day" and the days of Týr and Thor became the prosaic "third day" and "fifth day".