"Dydd Mercher, Dydd Iau"
It all comes from the Roman week days. Dies Lunae, Dies Martis, Dies Mercurii, Dies Jovis, Dies Veneris, Dies Saturni, Dies Solis. It was later translated to the other languages of the empire, and in the case of germanic languages, the gods were changed to Nordic gods. Jovis to Thor in thursday, Mercury to Odin in wednesday, Sollum to Sun, Luna to Moon, etc. except in Saturday.