Here’s a tricky word combo I initially missed the other day while proofreading. One of those things I knew, but that I had never thought much about. It’s important however, “every day” and “everyday” have pretty distinct meanings. I did a quick Google search and from the number of hits I sure am not the only one who’s had trouble with this.
“Every day” (with a space) means something occurring or done each day, without exception. “Day” is the noun. “Every” is an adjective modifying “day.” This adjective is a special type of word called a Determiner and describes the quantity of the noun. Other adjectives similar to “every” include “each,” “many,” and “several.” Turns out there are many types of determiners, including the articles- a, an, the; and cardinal numbers- five, twenty, forty. Wikipedia has seventeen different categories. (Who the hell made English so dang complicated?!)
Ever day I drink too much coffee. “Every day” indicates how often I rot my stomach.
On the other hand, “everyday” (without a space) is an adjective. It means “ordinary” or “unremarkable.”
During the winter, shoveling my driveway becomes an everyday chore. “Everyday” describes “chore.”
The way to tell which to use is ask the question- can I replace it with “each day?” “Each day I drink too much coffee” sounds fine. “An each day chore” makes no sense.
Further Reading:
Wikipedia entry on Determiners
On Target, a communications newsletter from Purdue University

I will think about this everyday. This is an each day thought process.
Thanks for posting this. It’s one of my pet peeves! I see this mistake everywhere from newspapers to my shampoo bottle.
Thanks Sean. You really know you’re a writer when you critique the shampoo bottle!