Heating Degree Days are a measure of the coldness of the weather.
They are calculated by taking the average temperature on a given day and subtracting it from 65°.
Each degree the average daily temperature falls below 65° is a heating degree day.
Cooling degree days are also based on the day's average minus 65°. They relate the day's temperature to the energy demands of air conditioning.
For example, if the day's high is 90° and the day's low is 70°, the day's average is 80°. 80° minus 65° is 15 cooling degree days.