Everyday vs. Every Day
As an adjective, this adverb means common or informal. Everyday is perfectly acceptable in formal writing.
It is not appropriate to wear everyday clothes to one’s convocation.
This adverbial phrase can be replaced by each day. This term may also be used in formal writing. It is not synonymous with everyday.
Homework must be left in the professor’s mailbox every day.
Homework must be left in the professor’s mailbox each day.
Homework must not be left on the professor’s lectern everyday.
Because everyday cannot be replaced with each day, we know this usage is incorrect.