We (and his teachers) can't work out how he does it. I think at some point he's sat in front of a PC calendar and memorised it!
He can also recite the US and UK release dates for most kids films (he learns them from Wikipedia)... and then he can subsequently tell you the day of the week!!
Most of his tricks are based on his amazing memory, he recites long sections from film and TV shows years after watching them.
That doesn't mean his brain haven't developed the same algorithm for it.
The brain is exceptional at noticing patterns. Some people's brains are very preoccupied with numbers, and abstract data. (Some people like spotting license plates, some can't even look away.)
You can also check if your son has "Eidetic memory" . It comes in very handy sometimes ;).