Nothing in particular. That is, lots of stuff will happen, same as lots of stuff happens every day, but there's absolutely no rational reason to believe this date in particular will be different than any other.
No
Battle of Agordat happened on 1893-12-21.
Battle of Ferozeshah happened on -22-12-21.
Battle of Konya happened on 1832-12-21.
Chinchow Operation happened on 1931-12-21.
Siege of Weinsberg happened on 1140-12-21.
Battle of Sahagún happened on 1808-12-21.
No there is no indication that something drastic will happen on the 21st December 2012.
Battle of Kutná Hora happened on 1421-12-21.
Battle of St. Vith happened on 1944-12-21.
Kurnool train crash happened on 2002-12-21.
Capture of Hooly Honore happened on 1791-12-21.