What is Heating degree day, cooling degree day?

With $\theta_i$ the average of a day’s maximum and minimum temperatures in degrees Fahrenheit, the day’s heating degree days are $\mathrm{HDD}_i = \max(0, 65 - \theta_i)$ and its cooling degree days $\mathrm{CDD}_i = \max(0, \theta_i - 65)$; a period’s index is the sum over its days.