These two get mixed up every day because they sound the same; it’s an everyday problem.
Every day is a two-word phrase that is a noun (day) preceded by an adjective (every). So if you’re talking about the days themselves, and you mean all of them, then use the two words every day.
Example: I will pray for you every day until this crisis is over.
I find this exercise easier to do every day.
Everyday, on the other hand, is a one-word adjective that means daily, or commonplace, or ordinary. We often say something is an everyday occurrence, which means it happens (note the two following words) every day.
Example: Use the everyday silver, not the fancy stuff.
This kind of nuisance is becoming an everyday affair.
Just remember: the one word everyday has to describe something other than a day! If you’re talking about days, and you want to allude to each of them in a specific timeframe, then use every and day as two words.