The common mistake is to use "everyday" where you should use "every day." People make it when texting. A dozen businesses in my town make it on their store signs, menus, and painstakingly hand-painted window messaging. Corporations make it right in expensive TV commercials and billboards. But worse than that is the indifference: the habit spans the decades because it's seldom criticized or even recognized.
Yet, solving this requires no complex flow chart. It's simple. Here goes.
"Everyday," as one word, is an adjective. It means "happening every day" or "commonplace," as in "your everyday stroll," "his everyday routine," or "your simple, everyday plyin' shears." (Yes, plyin' shears. If you haven't plied with shears, then they may not be your simple, everyday plyin' shears.)
"Every day" is a common phrase, but it's just an everyday case of two words that happen to be used as a phrase. "Every" is a determiner, and "day" is the word whose relationship to the sentence it specifies, as in "Billiam visited the Heckin' Café every day. Not just some days, not once every four days, but every day." Grammatically, "every day" is identical to "each pear," "no volcano," "some planets," or "that clam." Unsurprisingly, each of those examples is a two-word phrase. Even if you hadn't known what a determiner was, your intuition likely enables you to feel the relationship of the phrase's first word to the second, and you can feel that relationship between "every" and "day."
This illustrates the mistake's impact on readers: once you understand the difference, reading "we're open everyday" feels like reading "Francine went to the orchard and inspected eachpear," or "Jess Phoenix decided novolcano should go unstudied," "perhaps someplanets have atmospheres like Earth's," or "if only thatclam could win the World's Finest Clam award." All who misuse "everyday" are constantly doing that without realizing it.
That's it. If you missed it in grade school, two or three minutes to learn the difference and you're set for life.