This could be it, but I have to think that there are earlier records somewhere: On April 21, 899 BCE there was a solar eclipse in China that was written about in the historical records. The historical records contain lots of information about various events that occurred, including events before this eclipse, but we could not with accuracy pinpoint how X day in the reign of so-and-so translated to the Gregorian calendar. But this eclipse, being rather distinctive, allowed scholars to pinpoint the day it occurred, with confidence that it was not any other eclipse at any other time that was being mentioned, and therefore get a better idea of when other events mentioned in the records occurred. It's distinct because the eclipse occurred at dawn. The eclipse was not predicted, and the events puzzled the observers, because from their perspective, it began to dawn, got dark again, and then dawned again. So it was called "The Day that Dawned Twice" and observers did not realize it was an eclipse. In 1987, astronomers investigated and calculated that there was an eclipse at dawn in the place mentioned that would exactly explain what was described by contemporary observers in the records, so that was what the writer was referring to. Here's an article: http://articles.latimes.com/1987-01-25/local/me-5800_1_earth-and-moon

Also: there is an old Chinese story of a solar eclipse that occurred over 4000 years ago: As the tale goes, the court astronomers Hsi and Ho did not predict an eclipse and as a result were executed. It has been suggested, based upon current calculations of the time and locations of eclipses, that this happened October 22, 2134 BCE, which is 4148 years ago. However, this is fishy. The story is likely entirely a folktale that only recently an astrophysicist, not understanding the Chinese context of the story as folktale, attached to that particular eclipse based upon calculations. And there are others who do not feel able to pinpoint the eclipse referenced (if it even happened, but, as I said, the events described likely never happened) as being THIS particular eclipse, and they instead give the date range of 2159 to 1948 BCE, which is a pretty big range.

I don't know a specific answer, but it will almost certainly be an astronomical event that we can, with modern astronomy, calculate when it must have been, instead of somebody mentioning that something happened on a day, and we know enough about that calendar to figure out when that day was in our own calendar. Eclipses happen rarely enough to be commented on, but so regularly that we can figure out all of them have happened in the last X thousand years, and predict them for the next X thousand years, too. Also comets that come around regularly (specifically Halley's) are often mentioned. I don't know if those count as "historical events" for your question, but they're the sort of thing that might be seen as a sign from the gods, and therefore influence peoples' actions.