This app helps you train the Doomsday Algorithm which allows you to quickly deduce the day of the week to any given date within seconds.
It was invented by John Conway in 1973. Here, a variant is used which was found by Fong and Walters in 2010.
For more information and an in-depth explanation of the algorithm please see the Wikipedia article. But the app contains a condensed explanation and also detailed practice modes for the three major steps in the algorithm.