Everyday vs Every day

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Everyday is an adjective used to describe things that (1) occur every day, or (2) are ordinary or commonplace. In the two-word phrase every day, the adjective every modifies the noun day, and the phrase usually functions adverbially. 

For example:

Every day you eat breakfast.

You brush your teeth every day. 

Maybe you go for a walk every day. 

These are everyday activities.

Helpful Tip:

When you’re not sure which one to use, try replacing everyday/every day with "each day".

If "each day" would make sense in its place, then you want the two-word form. 

I eat an apple each day of the week.

I eat an apple every day of the week.

Everyday, meanwhile, is synonymous with daily or ordinary, depending on its sense.