Everyday vs. Every Day

One common grammar mistake which is almost never mentioned is using the wrong "everyday". Whether you write it as one word or two completely changes the meaning. 

"Everyday" is an adjective. It describes something (usually a situation). Something that is everyday is common. It's so common as to be unremarkable. It is an everyday mistake. 

"Every day" is a periodicity. It describes how regularly something happens. People make this mistake every day. 

Quick example:

I eat lunch every day. 

My turkey sandwich is an everyday lunch.