Everyday vs. every day

My husband and I sometimes eat at Wild Honey. On our last visit, I was struck by this error on their new menu:

OPEN EVERYDAY FROM 9AM

It should say:

OPEN EVERY DAY FROM 9AM

The space between “every” and “day” is missing.

Now, you may be thinking, “Hang on, ‘everyday’ is a perfectly good word!”

Yes. Yes, it is, but it’s an adjective, and what’s needed in this and similar contexts is the two-word adverb phrase.

Here’s an example showing how to use the one-word adjective in front of a noun and the two-word adverb at the end of a sentence to modify the verb:

These are my everyday shoes. I wear them every day.

Now, can anyone tell me why there’s no such word as “everywhen”? We have “everywhere”, and “everything”, not to mention those vaguely plural singular words “everyone” and “everybody”.