Great Party Stunt:
"Name any date, and I'll tell you what weekday it falls on."

The single most common characteristic of autistic savantism is the ability to calculate calendars.  Name any date - in any century - and almost instantaneously, the autistic savant person (formerly referred to as an "idiot savant") can name the day of the week on which the date falls.

Interestingly, most savants cannot even tell you how they do this.  They do not understand the workings of their own minds well enough to explain any algorithm or process by which they perform the calculation.

I recently saw a demonstration of this phenomenon on a television documentary.  While I am far from being a savant, I did spend a few hours noodling the phenomenon and arrived at a method by which a person of "normal smart" intelligence can perform mental calendar calculations, typically in a range from 15 to 30 seconds.

This is a great stunt to pull in any "I bet I'm smarter than you" contest.