"The concert is not on Sunday."
Translation:El concierto no es el domingo.
Because that's not how it's said in Spanish. They say "es el domingo" or "no es el domingo."
Prepositions and article use (the, a) vary a lot between language to language. Prepositions often express really abstract things -- why do we say a game or appointment is "on" a day? A day isn't a table or a shelf! It's just an idea, a chunk of time with a convenient label.
It's tempting to think that "en" means "on", but it really doesn't. Sometimes it's used like we use "on", sometimes it's used like we use "at" -- "el chico está en la mesa" "the boy is at the table" -- and sometimes, like here, it isn't used at all.
It's just it's own thing; you just have to learn how it's used in Spanish.
When a day of the week follows a form of the verb ser, no article is used. For example: Today is Monday (Hoy es lunes)
This link will help you more: https://www.spanishdict.com/guide/using-the-definite-article-in-spanish
I cannot seem to "guess" whether to use "es" or "está." So frustrating! I tell myself if it's a permanent thing like nationality, or a quality like "intelligente" it's es. "Él es americano." And for mood or health or location, it's está. "Él está acquí." And yet, I'm wrong about half the time! Like this time. I used está in this sentence: "La fiesta no ____ el domingo" and it was wrong. And yet it's not a permanent thing. I'm just not seeing it.