Pluto Day
The story of how Pluto was discovered actually starts in the 1840's, when one Urbain Le Verrier determined that there was a planet outside of Uranus, but that planet obviously wasn't Pluto, it was Neptune. But the same methods by which Neptune was discovered led to another beyond it. You see, Uranus was demonstrating some oddities in its orbit, oddities caused by its nearest, yet undiscovered, neighbor, Neptune. Once they were able to actually observe Neptune, they realized that another planet must be disturbing Uranus's orbit as well, what they were seeing couldn't be explained merely by Neptune.
This led to a search for Planet X (an Amazing name that we think Pluto should have kept, but we're not able to do anything about that, obviously) headed by Percival Lowell. Unfortunately Powell would pass from this mortal coil (and into the hands of Pluto, God of the Dead) before Pluto was discovered… At least, before he would know about it. You see, during their surveys of the deep sky in search of 'Planet X', two faint smudges would appear that were later to be revealed to be Pluto. The actual discovery of Pluto happened in February of 1930 by Clyde Tombaugh.