Krystal is right, "todos los días" would be "everyday", and "cada día" would be "each day". I'll give you some examples off the top of my head: "Vas al gimnasio todos los días? No, no todos los días. El domingo descanso." ( Do you go to the gym everyday? No, not everyday. I rest on Sundays. ) A funny example from an average parent: "Todos los días es la misma cosa! Nunca recoges tu ropa sucia!" ( Everyday it's the same thing (or story) with you! You never pick up your dirty clothes! ) "Ya pasó lo peor de la enfermedad. Cada día que pasa me siento mejor." ( The worst phase of the illness is over. Each day that passes makes me feel better. ) "Recuerdo cada día que pasamos juntos." ( I remember each day that we spent together ) I would just add that if you use them interchangeably, the idea will be understood, because they are quite similar.

What Krystal said.

I would translate them slightly differently. "Todos los días" is "all the days," so something like "every day." "Cada día" is "each day." Very similar, but a different nuance.