Every Day means each day. Everyday
means commonplace, ordinary, typical.
Here are some examples for using every day and everyday correctly:
Jane takes her dog out for a walk every day. It is important to
floss every day.
Jack did not take very good care of his everyday shoes.
An "everyday occurrence" does not necessarily mean it occurs every
day. It only means means it is an ordinary, commonplace
occurrence. It is not something unusual. Everyday is an adjective,
so it describes an attribute of the occurrence.
If something occurs daily, you say it "occurs every day" or that
it is a daily occurrence. Since "every day" is an adverb, it
cannot be used as an adjective to describe the occurrence.