- every day: every (determiner) + day (noun)
= 'every' is used with singular nouns to refer to all the members of a group of things or people (every day, every minute, every student...). So it means each day.
'His father bikes to work every day.'
'Ever day seemed the same to him.'
'We work every day except Sunday.'
- everyday: is a single word and is an adjective, so it's the one that is used in front of a noun to describe something as normal or commonplace.