It only happens the first twelve years of every century that has ever been. It happened last year and every year before that back to 2001. It happened from 1901 to 1912, 1801 to 1812, 1701 to 1712 and on and on.
So special. Aaaaaiiiieeee! And we’ll never see it again! /s
And because of the Mayan Doomsday, we will not get to next year with its 13/13/13!
But we only got one 12:12:12 that was in the year 1212.
The most important event of that year was the Battle of Las Navas de Tolosa, the most important turning point in the reconquista.