A student asked me about this a couple days ago. It’s one of those grammar rules I feel like I know inherently simply from being a voracious reader, but we took a moment in class to pinpoint the distinction between “every day” and “everyday.”
“Everyday” is an adjective. We use it to describe a noun. According to Merriam-Webster, it means “encountered or used routinely or typically : ordinary.” For example, “The sun rising in the east is an everyday occurrence.”
In other words, if you can substitute the word ordinary, you want everyday. If you can substitute the two words each day (or per day), then you want every day.