The story
Tuesday belongs to Tiw, a Germanic god of war and law, known in Norse myth as Týr. The Romans called this day Martis dies, "day of Mars", their war god — hence French mardi and Spanish martes. When Germanic speakers adopted the Roman week, they swapped Mars for the god they considered his equal, and Tīwesdæg was born: "Tiw's day".
The "-es" in the middle is the old possessive ending — the same one that survives as the 's in Tiw's. It is why Tuesday, Wednesday and Thursday all contain an S that sounds like a Z.
The spelling ue is a Middle English way of writing the sound that grew out of Tīw: the vowel and the W merged into a "yoo" sound, written ew or ue. British English still says "TYOOZ-day". American English dropped the "y" glide after T, D and N — the same change that gives Americans noo for new and dook for duke — so the American word is TOOZ-day.
