Easy to remember – Monday is the Moon Day. The word moon is associated with the Norse god Mona (Máni). He was pulling the moon accross the sky, trying to escape and save it from a mythological wolf. At the end of the world, the wolf will catch them and tear the moon into pieces. This apocalyptic period of the world is called Ragnarok in Norse mythology.
In Romance languages, Monday is also the Moon Day (lunes in Spanish, lundi in French, lunedi in Italian: coming from Latin Dies Lunae).