Every day

A common spelling mistake is to write "everyday" as one word when we want to say "todos los días". Note that this phrase is just like every morning, every Tuesday, every year, every night, every woman, every child:
EVERY + singular noun = todos los ...(días, mañanas, martes, años, noches, mujeres, niños).

Do not say all the days or all the mornings, all the years, all the nights.

ALL DAY (singular!) means todo el día, no todos los días and all days is not correct.

He plays tennis everyday.
He plays tennis every day (TWO WORDS!) = juega al tenis todos los días
He plays tennis all day = juega al tenis todo el día


There is an adjective EVERYDAY (one word), but it has a different meaning (= cotidiano, corriente) and you are not very likely to come across it (=es improbable que os topéis con él):
A dictionary for everyday use = un diccionario para uso diario