The names of the day of the week were coined in the Roman era in Greek and Latin, in the case of Monday as ἡμέρᾱ Σελήνης, or diēs Lūnae ("day of the Moon"). Many languages use either terms directly derived from these names or loan translations based on them. The English noun Monday derived sometime before 1200 from monedæi, which itself developed from Old English mōnandæg and mōndæg (literally meaning "moon's day").

Source: Wikipedia