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I think you're right with it being after Mars..... but Mars as in the Roman god of war. Since french is from latin and english is from german, it makes sense that they have two different words named after the same thing.

Tuesday -- Tiu's day

Middle English tiwesday or tewesday
Old English tiwesdæg "Tiw's (Tiu's) day"
Latin dies Martis "day of Mars"
Ancient Greek hemera Areos "day of Ares"
Tiu (Twia) is the English/Germanic god of war and the sky. He is identified with the Norse god Tyr.

Mars is the Roman god of war.

Ares is the Greek god of war.
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