The classic way of estimating heating load in smaller, envelope-dominated buildings is to assume the rate of heat loss is proportional to the difference between the inside and outside temperature.
The “design” heating load is estimated based on an outdoor temperature that is close to the lowest outdoor temperature for the location based on long term weather records. That temperature is called the 97.5% design dry bulb temperature. The reason heating load is not based on the 100% lowest outdoor temperature is because such a condition is very “short-lived.” It might only exist for one hour during one of several consecutive years. Basing the design heating load on such an infrequent and extreme temperature would lead to significant oversizing of the building’s heat source, especially if the designer hedges their estimate by including a generous safety factor. Furthermore, most buildings have sufficient thermal mass to “coast” through a short duration extreme low temperature when and if it occurs, using equipment sized for 97.5% design dry bulb temperature.