Might want to try a new battery with more cold cranking amps. Last week temps dropped to about 8 below zero. The battery in my 96 camaro is about 3 years old and it barely started the car. Even my wife's 02 pontiac was a little cranky. Your battery may have enough power to start the car when its warmer, but once the temp drops so does it's cold cranking amps. If your battery is not new, you might want to get a new one.
1969 Camaro 350, Trickflow heads, ST-10 4spd, Hotchkis suspension, Baer SS brake kit, moser 12bolt