"I need the letters for Monday."
Translation:Yo necesito las cartas para el lunes.
Because you're talking about a specific day of the week here - Monday, rather than Mondays in general which wouldn't need a preposition.
In Spanish, on the other hand, when you want to talk about something happening on a specific day of the week, you must use a definite pronoun before the day of the week. The days of the week are always masculine, so you’ll use either el or los.
When talking about one specific day of the week, use el.
Iré a Bogotá el lunes que viene. (I will go to Bogota next Monday.)
When talking about something habitual, or something that happens every week, use los.
Voy al gimnasio todos los martes. (I go to the gym every Tuesday.)
Because "para el lunes" means "for Monday" and "del lunes" means "of Monday".
I typed "Yo necesito las letras para el lunes." I got it wrong. Is there a context issue here? Maybe they needed the ABCs for monday
why when I hover over "the" the pop-up shows "el" instead of "las." I typed in "Yo necesito el cartas para lunes." It marked me wrong even though I used what was shown in the pop-up. I still would have had it wrong for not putting "el" in front of lunes. However, why can't the pop-up show the correct word to properly use?