See this support article:
https://support.mozilla.org/en-US/kb/how-print-websites#w_margins-and-header-footer

Well, I see where it shows how to get the date and time, which I already knew, but I don't see where it shows how to get the date without the time.

As far as I can tell, there's no built-in option to get the date printed without the time. It's hardcoded in a C++ file.

As a workaround, you can enter custom text for the footer, but it would need to be updated every time the date changed. I can think of a couple ways to automate that:

• An add-on (but I'm not aware of any add-on for this)
• External process to push a new date into prefs.js, a settings file that stores your header/footer preferences, among others (but Firefox should be closed when you update this file, and it will be read when you restart)

Not easy enough, I know.