Everyday is an adjective meaning "daily."
Every day is a time expression meaning "each day" or "regularly."
Note the difference in the following sentences:
1. Jane goes to class every day (each day)
2. Jane has an everyday class. (a daily class)
The most common error is writing "every day" as one word (everyday) as in the following sentences:
1. I used to jog three miles everyday (Incorrect)
2. I used to jog three miles every day. (Correct)