People write everyday when they mean every day. It’s quite simple: If it’s an adverb, telling when, then it’s two words: Every day I eat a big helping of lingonberries. When do you eat your lingonberries? Every day. Every single day. By the way, I don’t have much cupboard space, so driving to Ikea to buy the lingonberries is part of my everyday routine.
From the second part of the example, we see that written as one word, everyday is an adjective, modifying a noun. So, if it precedes and modifies a noun, it’s one word.
If it simply precedes a noun, it might not modify it, so be careful. Consider this: Every day squirrels pillage our bird feeders.
The modifier tells when the pillaging occurs, not what sort of squirrels are pillaging. A comma helps clarify: Every day, squirrels pillage our bird feeders.
Maybe the best way to have this stick in your head is through a couple of popular songs: “Every Day I Write the Book” by Elvis Costello, and Sly and the Family Stone’s “Everyday People.”
From the former:
“And I’m giving you a longing look;
Every day, every day, every day I write the book.”