The one-word modifier everyday and the two-word phrase every day are not interchangeable (despite store ads that say, "Lowest prices everyday" – incorrect).
Everyday (one word) is an adjective meaning "encountered or used typically or routinely; a synonym is ordinary.
Every day (two words) literally has the same meaning as "each day."
A simple way to test which is appropriate is to substitute "each day" in place of "every day / everyday." If "each day" works, we want to use every day (two words); if "each day" does not work, we want everyday. For example, "We have low prices every day" = "We have low prices each day"; therefore, every day is correct – and everyday is incorrect. On the other hand, since we may not correctly rephrase "This is an everyday event" as "This is an each day event," the one-word adjective everyday is correct.