|Image obtained from the National Park Services|
On Sunday morning, December 7, 1941, the Japanese attacked the American Pacific Fleet at Pearl Harbor. The Empire of Japan had not previously declared war on the United States; a lengthy ultimatum was being decoded and typed up at the Japanese Embassy in Washington even as the 353 Japanese fighters, bombers and torpedo planes rained down devastation on Hawaii, killing 2,403 Americans, including 68 civilians; 1,177 died on the U.S.S. Arizona alone.
You may vaguely recall that American General Billy Mitchell predicted a Japanese attack on Pearl Harbor in the 1920s. But the Japanese took particular inspiration, in drawing up their battle plans, from the British attack on the Italian Navy at Taranto in November 1940.
In addition to the linked Wikipedia articles, you may find the National Park Service site for the Arizona Memorial and related Pearl Harbor Historic Sites helpful today as you remember Pearl Harbor. Resources for teachers and students are linked from that site as well.
Other observances today are not nearly as significant, although today might be International Civil Aviation Day -- but that's a link to a reference to a 2011 observance; it doesn't seem to be on the UN calendar this year.
Of course, anyone who's flown recently knows that air travel is less civil than ever.
Usual Suspect gone-ta-pott.com suggests that today is Teacher Appreciation Day; several of the Usual Suspects say today is also Letter Writing Day. Combine the two by writing a letter of appreciation to a favorite teacher.
But don't write your letter while celebrating National Cotton Candy Day (also today); you'll just make a mess.