Provide F10 keyboard shortcut to toggle main menubutton, and a keyboard shortcut to toggle the calendars menubutton?
Most/all GNOME apps use the de-facto standard F10 keyboard shortcut for toggling the main menubutton, so GNOME Calendar should as well.
But GNOME Calendar also has an important menubutton next to the main menubutton: the calendars menubutton. Perhaps this one could have the F12 shortcut, or F8? (F9 is usually for toggling sidebars, F11 for fullscreen).