For some reason, everybody--or at least, a lot of the bodies--has come to the conclusion that everyday is always one word, no matter what you're doing with it or why.
Everyday is an adjective, meaning it describes a person, place, or thing. It means "ordinary, used routinely," according to Webby. Often, you'll see it used in the phrase everyday life:
For Buffy, killing vampires is just a part of everyday life. That's not the case for most of us.
The peeve comes in when folks use everyday when what they mean is every day--meaning "continually, day after day":
Every day, Buffy finds a demon or vampire who needs vanquishing.
Every day, I find a dirty sock that needs washing.
My everyday life doesn't look much like Buffy's, I guess.
See the difference? If you can't put a noun after your use of everyday/every day, split it into two words.
I knew you'd get it. Now I won't have to come after you with Mr. Pointy.