John Horton Conway is an English mathematician who came up with an algorithm to figure out the day of the week for any date, past or future, in a mere seconds. In this video, Joe Hanson of It’s Okay To Be Smart, shows you how you can do it too – all with just one hand. All you need is a little bit of practice.

Via – It’s Okay To Be Smart
Further Readings And References @ John Horton Conway (Wikipedia), Doomsday Rule (Wikipedia) and Nature