The word “everyday” and the phrase “every day” are not interchangeable. Everyday is an adjective which means “encountered or used typically or routinely”, or “ordinarily”. Where “every day” has the same meaning as, “each day.”
You can test which is the correct use by substituting “each day” in place of “every day / everyday.” If “each day” works, then use every day (two words); if “each day” does´nt work, then use everyday.
“We have low prices every day”, is the same as “We have low prices each day”, therefore, every day is correct.
“This is an everyday event”, is not the same as “This is an each day event”, therefore, everyday is correct.