"Odio los lunes."
Translation:I hate Mondays.
Say the direct translation, then ask yourself if that's how you would say it in English.
"Odio los lunes" = "I hate the Mondays." --> Do we say "I hate the Mondays" in English? No, we just say "I hate Mondays."
"No leemos durante la cena." = "We don't read during the dinner." --> Does it make sense to say "We don't read during the dinner" in English? Nope, we just say "We don't read during dinner."
In my experience with questions and such, you use the article+plural form when you're talking about all of them as a group, but you use modifiers if they're referring to specific dates/things. In English we do drop the article, but on the other hand we don't usually say "that person knows the cars" unless we mean specific cars. We would say "that person knows cars". That's not including different verbs one can use, of course.
There seems to be a lot of things in Spanish that are contextual, and vocabulary differ from region to region. Just have confidence you'll adapt if you actually go somewhere.
I think in this sentence 'los' is used to indicate that its plural. The days of the week don't have a separate plural form in Spanish. So without 'los' this could also have meant "I hate Monday". Whereas after putting 'los' its clear that its plural so its "I hate Mondays"
You always have to use the article when using words such as Gustar, Odiar, Encantar, to describe things in general.. If you want to say 'I like monkeys' You MUST say 'me gustan los monos'.. (Even though it's just referring to monkeys in general) It's just the way it is! Languages are not always logical..