On the top app bar, imagine an entry to the left of today's date that lets you pick any date in the past by using a calendar control. You can use one of the services on the
Windows Azure Marketplace
to pull historic weather data.
As a next step, you could mine historic data to produce predictive forecast modeling, or create toast notifications that contain interesting weather-related facts ("On this day in 50 years ago, the coldest temperature was recorded in your hometown".)