Below freezing temperatures would be considered to be to cold for the human body to sustain for too long.
So if the weather is below freezing outside you should not stay outside for very long and should take frequent breaks and come inside where it's warm.
You should also avoid being outside too often and if possible you should stay inside the house during below freezing temperatures to avoid freezing to death or getting hypothermia.
Our normal core body temperature is 98.6 F and if your bodies core temperature drops to 95 F then mild hypothermia can start to set in and anything below 95 F then severe hypothermia begins to set in and you risk freezing to death.