Japanese attack on Pearl Harbor.
because its pearl harbor
You may be thinking about the famous speech by US President Franklin D. Roosevelt before the US Congress on December 8, 1941, following the Japanese attacks on Pearl Harbor, Hawaii (and the Philippines) the previous day.
A day in December.
Traditionally it has been the US Civil War and WW2.
#
just one? Civil War
George Washington
On December 10th, Mississippi is entered in to the US as a 20th state.
From a perpetual US calendar: December 8, 1992 was on Tuesday.
From a Perpetual US Calendar: December 28, 1972 was on Thursday.
From a perpetual US calendar: December 22, 1960 was on Thursday.
From a US perpetual calendar: December 22, 1992 was on Tuesday.