There’s a big sale at the mall today.
I think that “Rebajas” translates more closely to “discounts” or “price reductions” so that the more accurate translation into English might be: “Today there are big discounts at the mall.” For the singular: There is a big sale at the mall: “Hoy hay un gran venta en el centro comercial.”
“Rebajas” does mean “discounts” or “price reductions.” But the English translation’s just fine, meaning-wise. “Sale” is often used when the store has reduced prices or discounts for multiple items, and the relevant definitions for “rebaja” in the Real Academia Española’s dictionary seem to require the plural:
- f. pl. Venta de existencias a precios más bajos, durante un tiempo determinado.
- f. pl. Período de tiempo en que tienen lugar las rebajas.
In that case, “grandes” would have to be used to ensure agreement between the noun and adjective.