This is the 803rd most frequent Spanish word.
aquel
"That" (referring to something distant in space or time).
The word 'aquel' is used here as a demonstrative adjective to refer to a day in the past, conveying a sense of distance in time from the present.
Aquel día fue inolvidable para todos nosotros.
That day was unforgettable for all of us.
In this sentence, 'aquel' functions as a demonstrative adjective qualifying the noun 'man', indicating a specific individual somewhat distant in thought or context.
Yo recuerdo a aquel hombre que conocimos en la plaza.
I remember that man we met in the square.
Here, 'aquel' acts as a demonstrative pronoun, identifying a specific book distinct from other possible referents.