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 reprint it in the second edition of the book. 'Cause I can do that. :)
This phrase is pretty much what it sounds like: something that happens on a daily basis.
I brush my teeth every day.
Every day, I send my kids off to school.
I check the mail every day for another rejection.
It's easy to know whether to keep the space.
Just ask: Can I add the word "single" between "every" and "day" and have it make sense?
If so, keep the space:
I brush my teeth every single day.
Every single day, I send my kids off to school.
I check the mail every single day for another rejection.
They all work. Woohoo!
Everyday (one word)
Going all technical for a second, this is one word because it's an adjective. It describes what comes next.
Try replacing "everyday" with a different adjective, one that means something similar, like:
Does the sentence still work?
Running out of toilet paper around here is an everyday (normal/typical) event.
Her everyday (typical/regular) migraines are debilitating.
Is this type of outburst an everyday (normal/common) occurrence for your daughter?
If you notice, those kinds of words don't work as replacements for the two-word variety (every day):
I brush my teeth every day (typical/normal?).
Every day (regular, common?), I send my kids off to school.
I check the mail every day (usual, normal, typical?) for another rejection.
Ask: Can you replace the phrase with a word such as regular, typical, normal, common, or usual?
If YES: Make it ONE word, no spaces (everyday). It's an adjective.
If NO: Use TWO words and a space (every day). The phrase is just referring to a regular time period.
If you've decided the phrase needs a space, test it further by adding "single" between "every" and "day," making it, "every single day." Does it still make sense? If so, you're good to go.
See? Easy peasy.