Generally, they are people who have very poor intelligence, but are exceptional at something.
Like one guy is unable to multiply 3 x 5, yet he could tell you that February 7, 1900 fell on a Wednesday in a few seconds. Or that December 8th, 2098 will fall on a Monday.
Well, not necessarily poor intelligence. But I think those are the coolest cases. If you've ever seen Rain Man.
This is about Kim Peek, who Rain Man was based off.
I'm impressed that you actually made sure those weekdays were accurate
Yeah it's interesting that lots of above average intelligence people have other parts of their brains severely below average.