I wrote about this issue nearly a year ago. And still, it persists.
Everyday means ordinary or commonly occurring while every day means each and every single day. Sly Stone assures us that in spite of his fame and lifestyle, he is simply “everyday people.” Elvis Costello, unfortunately, gets it wrong when he sings, “Everyday I write the book.” Clearly he means “every day.” Who knows how many record sales this cost him?
What do I have to do? Throw myself off those rocks? Move on to something more substantive? (Right. Like that’s gonna happen …)
And, yes, technically this is not a matter of grammar. But “usage” doesn’t have quite the same ring to it.