Tuesday...

Tyr or Tiw was the Norse God of War. When the Germanic Angles and Saxon's invaded England in the 500's they suplanted a culture that had been heavily influenced by Rome for several hundred years. The day, Tuesday, had already been named for the Roman God of War, Martius (notice in French, Italian, and Spanish - the word for Tuesday is still derivative of the Roman God - Mardi, Martedi and Martes - respectively). When the Germanic tribes conquered England, they laid their own lexicon over that of the Roman's so that the Norse God of War now supplanted the Roman God of War (after all the Norse God was obviously more potent). Thus they called the day of the God of War tiwesdaeg.