The earliest event in human history for which a definite date is known is a battle between the Lydians (allies of the Greek Spartans) and the Medes (ruled by the Persian king Cyrus) who had been locked in a war for five years. As the two sides faced each other for a crucial daytime confrontation, a solar eclipse occurred. This was taken as an sign of the gods' disapproval and the Lydians and Medes agreed to end the fighting then and there. The dates of solar eclipses can be figured out with great accuracy and this one is known to have taken place on May 28, 586 BC.
Much less certain is the birth date of Christ. It was not until AD 440 that Christmas was celebrated on December 25. This date was chosen because it coincided with the birth date of Mithras, the Persian sun-god, and was close to the pagan festival of Yule. In AD 534, Dionysius Exiguus (also known as Dennis the Little), created the system, still used today, of counting the years from the birth of Christ. Unfortunately, he slipped up in his calculations. No one knows exactly when Jesus was born, but it was probably around 6 BC and certainly before the death of Herod the Great in 4 BC.
As for the future, there's no shortage of predictions about the end of the world. According to the Mayan "long count" linear calendar, it will happen on June 5, 2012.
Other calendric curiosities: February 1865 is the only month in recorded history not to have a full moon, and months that begin on a Sunday will always have a Friday the 13th.