What are the most important events, in your opinion, that have happened on your birthday? Things in your own life don't count (so no "I was born!")?
The two most important things that have happened on my birthday, October 28, are the Battle of Milvian Bridge in the year A.D. 312, in which Constantine defeated Maxentius and, more importantly, supposedly saw the sign that would lead him to convert to Christianity, and the end of the Cuban Missile Crisis in 1962—it was the day on which Khrushchev announced that he would withdraw the Russian nuclear missile bases.
Wikipedia is very good for this, since it has a comprehensive listing of events that have happened on every day of the year. I didn't know either of these things fell on my birthday until I looked it up.
Also, just for fun, the most important people (known to be) born on the same day as me are Bill Gates and Mahmoud Ahmadinejad.