to budget, here is a list of dates to avoid due to premium pricing for Disney resorts posted by Mousesavers.com.
The periods to avoid in 2012 are listed below:
* New Year - January 1
* Marathon weekend - January 5-7
* Martin Luther King, Jr. weekend - January 13-15
* Peak Season/Presidents Day weekend - February 16-25
* Peak Season March 9-31 (especially weekends)
* Easter - April 1-14
* Summer at the Value and Moderate resorts - June 1 - August 4 (especially weekends)
* Fall weekends - September 28 - December 20 (especially Columbus Day Weekend, October 5-7 and Thanksgiving weekend, November 19-22)
* Holiday Season - December 21-31
http://www.mousesavers.com/disneyresorts.html#offseason
These dates can also be used as a proxy for crowd levels. Disney knows when demand is high and extracts a premium.
There are a number of sites that have historical weather information and the information is presented in different ways. Here are a few.
http://www.weather.com/outlook/health/fitness/wxclimatology/monthly/graph/USFL0615
http://www.intercot.com/infocentral/weather/dailytemps.asp
http://www.plan-a-magical-vacation.com/disney-world-weather.html
http://www.*************.com/Temperatures and Attendance for Disney World.htm
One thing to remember about crowd levels is that Disney will adjust hours relative to crowd levels. For example, we usually go in June. It's common for the MK to close at 11:00 PM at the earliest with PM extra magic hours (EMH) going to 2:00 AM. We had one or two nights this year when it closed at midnight and one night it was open until 2:00 AM without EMH. We take breaks for swim or naps during the hottest parts of the day.
We go in June mainly because the kids are in school. If I had my preferences, I would probably go in November the week or two before Thanksgiving, right after Thanksgiving or the last week of April/first week of May. I would rather have a chance of hotter weather than cold weather.