Some people say that it's been long enough since 9/11 that the US should stop "remembering" it each year and move on. But in Britain we still annually celebrate the failure of the 5 November bomb plot by Guy Fawkes in 1605. That got me thinking: what is the oldest event that is still regularly commemorated or marked in some way?
This includes cases where the actual meaning of the event has pretty much been lost - for instance in the UK, Guy Fawkes Night these days is really just an excuse to set off a bunch of fireworks, but there are still connections to that original event.
Well, in Judaism, there's the Exodus from slavery, commemorated annually with the festival of Passover. However, there's no historical evidence that the Exodus took place, so it's possible it's commemorating a mythical event. Doesn't mean it didn't happen, but it might not have happened, so perhaps that doesn't meet the requirements of your question. However, if you are interested in commemorations of mythological events, then I should point out the Jewish New Year, which commemorates the creation of the world, which would have happened 5,776 years ago as of this Sunday.
The earliest Jewish event that is historically verified to have occurred and that is annually commemorated is the destruction of the first Temple in Jerusalem in 587 BC. That is commemorated with fasting every year, and is called Tisha B'Av. However, there are several other historical calamities that are also commemorated on that same day, such as the destruction of the second Temple, which was well within the historical period, happening in 70 AD. The events related to the Jewish holiday of Chaunkah happened in the 2nd century BC, and are commemorated annually.