An important thing to consider when using degree day data is where the data is coming from...you want consistent data season to season.
Data from a nearby aviation site would be what I would use.
I'm kind of a weather nut and so I use official data from National Climatic Data Center
Here is a direct link to the National Climatic Data access page http://www.ncdc.noaa.gov/IPS/lcd/lcd.html
....You'll see a little window with states in the middle of the screen...scroll to your state...click next...choose the station (these stations are most likely aviation weather station sites)...click next...choose a month or annual...click next....click on the link in blue.
All the climate data you will ever need will be shown.
Repeat these steps for whatever month or year you want
Here is a sample for Allentown for 2012. http://www1.ncdc.noaa.gov/pub/orders/IP ... 3BA51B.pdf
This link is broken, either the page no longer exists or there is some other issue like a typo.