The rise of Christianity
TBF, I remember this only because I have been reading D Dennis Hudson’s work.
The second would be the assassination of Caesar on the Ides of March 44 B.C.
Crucifixion of Jesus Christ in 30 A.D.
Battle of Thermopylae, 480 B.C.
I'd point out that we often don't know the exact historical date, that far back. For example, no one knows the alignment of the ancient Mesopotamian calendars with our own. So we know the exact Assyrian date of when Sargon II fell in battle during his last campaign, but it's a bit of guess as to what date in our calendar that was. Probably around 700 BC.