Car camping at the base of the Sangre de Cristo range, CO, in December of 1974. It was -20*F, and I was fine, slept like a baby. The next night I solo-skied a couple of miles down from Molas Pass, had a fire outside the tent door, bulked up on calories and hit the sack. A few hours into the sleep, I woke up with stage 1 hypothermia, shivering uncontrollably. I managed to pound down a bunch more food, I quit shivering, and decided to try sleeping again, rather than ski back up to the pass at 3 AM. My thermometer indicated it had dropped to -36*F. I felt lucky to have stayed alive. That's when my friends first realized that I was pretty crazy.
The coldest ever was a full week at Christmas of 1968 in Montana, where the temps didn't get above -48*F. No camping or hiking during that spell, but everyone kept doing what they usually did around town. Lots of dead car batteries though, and sparrows died overnight while perched on the power lines.