What is the difference between "everyday" and "every day"?

Solution 1:

"Every day" is a phrase that functions as an adverb, as in:

He does this every day.

"Everyday" is an adjective that means "encountered or used routinely or typically":

These are my everyday clothes.

People sometimes erroneously write "everyday" when they mean "every day", probably because they know they have seen "everyday" before, and don't realize there is a spelling difference between the adverbial phrase and the adjective.

Solution 2:

"Everyday" is an adjective meaning commonplace, used all the time (i.e., every day). "The medicine cabinet was filled with everyday remedies like aspirin and Tylenol."

"Every day" means something that happens every single day. "The sun rises in the east every day." That means you can count on that to be true, without exception, every day.

The two should not be confused, although as you note some people do confuse them. Some people also write "your" when they mean "you're" ...