I think your "heating days" and "cooling days" are a shortening of the terms "heating degree days" and "cooling degree days". Definitions:

Heating Degree Days
The cumulative number of degrees in a month or year by which the mean temperature falls below 18.3°C/65°F.

Cooling Degree Days
The cumulative number of degrees in a month or year by which the mean temperature is above 18.3°C/65°F.

These are from:

http://www.worldclimate.com/define.htm

Now, what does all this mean? See
http://makeashorterlink.com/?Y1AB25218



TEd