"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