Is the "Dé" always necessary? Or is it only when saying the day on its own? (Or the first in the list, as with "Dé Céadaoin no Déardaoin".)
Well, if you look at Déardaoin", it actually contains the Dé, it's been written into the word; you'll never see Dé Déardaoin. It's necessary when saying things like "This Tuesday," or "On Tuesday."
Mar shampla Dé Luain means "On Monday (i.e., this coming Monday) but Ar an Luan means "On Monday" (i.e. On Mondays)
it's funny that i managed to guess "ar an luan" and this without peeking on the first try. are the days of the week in most european languages meant to be similar?
I was wondering this too! Luan and Máirt seem like strong cognates with, for instance, "lunes" and "martes" in Spanish.
Well, most Western European languages get their names for the days of the week either directly or indirectly from Latin. In the Germanic languages, the planets/gods were replaced with Germanic equivalents, dies Martis -> Tiw's Day (Tuesday), for example. Irish directly borrowed several of the weekdays from Latin. Spanish, of course, inherited the names from Latin