I'm confused, What is the lexical relationship between "Monday" and "Tuesday"?
I mean is the relationship hyponymy, prototypes, polysemy, homophones, metonymy etc?
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I think that the origin of the names of the week exemplifies their nature:
Although our days of the week are not named directly after the Roman gods, they are named after the equivalent Anglo-Saxon/Germanic pantheon of gods and display Roman influences. In fact, English is one of the few Germanic languages to reference the original classical Latin names for days of the week.
You can find interesting information in the link above. Anyway it looks like there us no "lexical" relationship among them but pure symbolism.