El día - sustantivo masculino
Why is día masculine?
2 Answers
Because this noun evolved from the Latin dies into día. The fact that the noun ends in an "a" does not necessarily make it feminine. Examples:
El atleta
El planeta
El economista
El panorama
El problema
El dilema
A lot of these words have Greek roots.
I hope this helps!
There are many exceptions that you just have to accept and memorize.