Is there a difference between “every day” and “everyday”?

When you do something daily, let’s say, then you do it every day. Every. Day. Two words. Two separate words. “Every day” means, refers to, and includes all days, just like “every cantaloupe” means, refers to, and includes all cantaloupes. You get the idea. A few examples:

  1. Ken and Terry go jogging every day.
  2. Does she work in the office every day?
  3. We make sure to eat a nutritious breakfast every day.
  4. Every day is a winding road.
  5. Every day at work, they tried not to notice the mob of demonstrators.

When something is commonplace, ordinary, mundane, routine, average, run-of-the-mill, plain, or typical, then it can be accurately described as everyday. One word. One. “Everyday” is an adjective, a descriptor. For example:
  1. After several months, the workers grew tired of the menial, everyday tasks and chores.
  2. Both accountants think this quarter’s profits will far exceed anything resembling the everyday numbers the company is used to seeing.
  3. I am everyday people.
  4. Alex said Saturday’s banquet calls for formal attire, so it’s a good idea to forgo your everyday wardrobe.
That’s all there is to it. In sum, every day is an adverbial phrase meaning “each day”, while everyday is an adjective meaning “commonplace”, “ordinary”, or “routine”.