That would, of course, depend on the person, culture, time period, and age of the person being asked, I think.
Many would say the birth and/or death of significant religious leaders like Christ, Buddha, etc.
Others would site dates of start and ends of famous wars...or the assassinations of famous figures like Lincoln, etc.
Being a woman with a feminist bent, the most important date in history would probably be August 26, 1920...the day we of the female persuasion started our march toward equality (which hasn't yet been reached, btw! :)
The bottom line is that there are a number of critical events and it's difficult to impossible to identify the one most important date. If Jesus had never been born, he would not have been able to die on our behalf - but his death and resurrection was the reason he was born, so I would have to agree that the date of his death is more important that his birthday, and the date of his resurrection is more important than the date of his death.
The advent of agriculture, the taming of fire, the development of iron tools and the revolution it brought to agriculture, the invention of writing - there are many milestones that could be presented!
I agree with all the above posts. It is certainly very difficult to pinpoint one date in all of history that would be designated as the most important. However, since the Roman emperor (Augustus?) who was in power when Jesus was born spent so much time tracking down the baby boy who would eventually dethrone him, would you not put down Jesus' birth instead of Jesus' death as the most important date...at least for Christians?
For me and a number of history scholars, the most important date is 1456, the year that Johann Gutenburg perfected his printing press. In the year 2000, a group of scholars were asked to evaluate the 100 most important people of the past thousand years. The number one choice was Gutenburg. Without the press, the wisdom and knowledge of the ages would not have been available to the masses who have benefited from it, including those of us who post here. Its invention was probably inevitable, but it was Gutenburg who first perfected it.
The earlier posts mention the death of Jesus and the genesis of agriculture. These are definitely important dates. Yet we ourselves would have little or no knowledge of them were it not for the printed word. So my vote is 1456.
I'll argue for an unknown date in prehistory. It would be the date on which the first human being purposely planted the first seed and began the practice of agriculture. This innovation underlies all of human civilization. Because of this, I would argue that this day was the most important in all of human history.
In my opinion the most important date would be April 21 753 BC, which is the founding of Rome. With Rome came the Latin Language, which is the root of many of today's most important languages, including French, English, and Spanish.
my opinion is when the first programmable computer was built in 1936, because that's when everything started like a chain reaction, and has lead us to having mini computers and touch screen phones.