in Red Hook, NY, 1965-1979. The all time low during that time was -26 degrees F (-32 degrees C). Most winters will have some days below -20 degrees F. However, these days are generally on calm days with clear nights -- after "radiational cooling". Gusty days with near 0 degrees feel much worse -- wind chill factor, of course. Once had a period of 2 weeks all below 0 degrees.
One February while camping with my son and other Boy Scouts and sleeping in a "lean-to" (i.e. essentially outside), it got down to 7 degrees F. That was not a problem. The mistake I made was leaving my boots outside the sleeping bag. They had been damp and froze solid and might as well have been made of steel. That's when you discover boots need to be pliable to get them on. A few minutes of baking in front of a campfire did the trick on the boots. The least fun thing was going potty in an outhouse. :)
Just keep adding layers of clothing until comfortable.