Everyday is an adjective; it describes a noun and answers the question what kind of, as in the old song I Am Everyday People (what kind of people are you?), so it normally comes right before the noun. Temper tantrums are everyday occurrences (occurrence is a noun) with toddlers. Worship is an everyday event (event is a noun) at our home. These are my everyday shoes means these are the shoes that I wear for no special occasions; they’re my general footwear. Everyday can mean commonplace, and it can also mean daily, but it when it means daily, it is in the adjectival sense.
Every day is an adverb, answering the question when, as in the other (relatively) old song Every Day I Write the Book. It means every single day, so if you can insert single between every and day, then you should use them as two separate words. My toddler has a temper tantrum every day (when?). We worship at our home every day (when?). I wear these shoes every day except for special occasions. Every day means daily in the adverbial sense.
What’s important to remember is that whether you use it as one word or two depends on its position in the sentence. Are you asking what kind of (a daily occurrence) or are you asking when (it occurs daily)?
Sherry
Everything Language and Grammar 