WNW: Every Day or Everyday?

Which do you use? When? What's the difference? Is there one?
The everyday/every day mix-up is easily one of the most common mistakes I see in my editing work and one of the most common questions I'm asked.
Kinda figured it made sense to address it here. I do mention it in There, Their, They're as well, and I think I do a pretty good job of it. But recently, I had a brain flash about how to explain it even better.
I'm hereby using that brain flash in this post and reserving the right to...