When you intend to use either an adjective (for the most part this will come before a noun, as in everyday life ) or a noun that means "ordinary occasions," as in part of the everyday , the one-word version is correct. Adverbial uses, as in We should eat fruit every day , and the noun use that means "each day," as in Every day is different , call for the two-word version. Thus everyday in every way means "ordinary in all respects," whereas every day in every way means "daily and completely."