Resources to determine temperature probability for a given location on a given date
The most basic way to measure probability is to look at historical 
records. There are several sources for this data, including: 


http://wunderground.com 
ftp://ftp3.ncdc.noaa.gov/pub/data/noaa/isd-lite/ 


This assumes no climate change. If you believe in climate change, you 
will need to factor that in.