Mr. Rewrite sees mistakes like these almost every day, but he doesn't want his blog to be overwhelmed by every day/everyday errors. Rather than ignore this problem or his crusade to correct it, he's decided to spin off a blog at everydaymistakes.blogspot.com.
Feel free to share your examples of everyday/every day errors.
For the record, every day is an adverb (e.g. Mr. Rewrite blogs almost every day.). Everyday is an adjective (e.g. This mistake is an everyday occurrence in the periodicals and Web sites Mr. Rewrite follows).
In the cases above and below, every day should be an adverb keying on a verb that's written or implied ("We OFFER the $2 beer special all day, EVERY DAY"; "This store is open these hours EVERY DAY.").