2014 Hyundai Accent - Old battery strategy

I have an old battery and it’s-20 outside. Is it better to wait until air temperature reaches at least 0 to try and start it?

Why? Have it jumped, drive to whatever store wish to buy a new battery, have the new battery installed. You only have a couple of days or you will have to wait until December 26 or 27.

It will start or not start . If it does not start no harm to your vehicle . If it does not start then as Purebred says have it jumped and get a new battery .

If the only two options available are to test-start the engine at -20 degrees, or wait until it is 0 degrees, were I in that situation I’d choose the 2nd option. More likely to start at warmer temperature.

If You have no need of the vehicle. I never had a problem starting any of my cars in sub-zero temperatures, that was with 10W-30 Dino oil back in the 1960s, Pontiac 389, Plymouth 361,
Pontiac 400 in the early 70s, since then have lived in warmer climates.