Everyday is an adjective used to describe things that (1) occur every day, or (2) are ordinary or commonplace. In the two-word phrase every day, the adjective every modifies the noun day, and the phrase usually functions adverbially. For example, every day you eat breakfast. You brush your teeth every day. Maybe you go for a walk every day. These are everyday activities.
When you’re not sure which one to use, try replacing everyday/every day with each day. If each day would make sense in its place, then you want the two-word form. Everyday, meanwhile, is synonymous with daily or ordinary, depending on its sense.
When Hirohito spoke, few could understand him because he used a language so formal that it was unintelligible to everyday people. [Washington Post]
His owner says he’s excited to go to work every day. [Calgary Herald]
Is the trend for using high-end ingredients in everyday recipes going to take off outside restaurants and TV studios? [Guardian]
In both our cases, every day spent off school was a day spent playing video games. [Sydney Morning Herald]