I have no problem with "even-numbered days" to mean even-numbered dates. I remember the "gas crisis" of the 70s, when, depending on your license plate number, you could fill up on either even days or odd days. Maybe this is American, but if someone said "I do X on even-numbered days" I'd simply assume he meant the 2nd, the 4th, the 6th, etc.
Yes, I was going to mention this same thing. Filling up on odd-numbered days. The 31st of the month was a free-for-all, officially both odd and even. And there was even a 75 cent minimum purchase requirement, to prevent people from just topping off their tanks.