Unfortunately, this word is now used incorrectly almost as often as "comprise" is. Here's a quick primer on when to use "everyday" and when to use "every day":
- Everyday. This word is an adjective. Therefore, it can only modify nouns. Consider:
He was an everyday sort of fellow. (modifies "sort")
By now, he viewed the anthrax scare as an everyday occurrence. (modifies "occurrence")
- Every day. This phrase always acts as an adverb, so it can only modify verbs, adjectives, and other adverbs. It's no different grammatically from "every week," "every year," "every picosecond," et cetera. Like so:
I go down to the store every day. (modifies "go")
Every day I write the book. (modifies "write") That wasn't so hard, now was it?
Oddly, rather the two being used indiscriminately (as is the case with "it's" versus "its"), "everyday" has more or less eclipsed "every day" among the illiterate. Here's why this is incorrect: Consider the sentence "I read the newspaper everyday." Here, the adjective "everyday" is being used to modify the verb "read." Now, you wouldn't say "This columnist writes good," would you? No, you would say "This columnist writes well," because only adverbs can modify verbs. So unless you're prepared to acquire a reputation among the cognoscenti for "talking good," you'd better wise up and start using "every day" when it's appropriate. (Yes, I know, that space takes a long time to type, but the rewards are rich indeed.) | 
