The moment humanity came into existence. Whether you believe the story as laid forth in the Tanakh literally, or you are an atheist who believes in pure random chance,
Evolution isn't "pure random chance," and since evolution is a continuum, it's not meaningful (except for a Creationist) to speak of the "moment" humanity came into existence.
Funny, though. I'd think, for someone who thinks it has something to do with Messiah Yeshua, it would be the moment of his death or resurrection, not his birth. But, to each his own.
No doubt, the feast of the resurrection was historically a far greater feast than the feast of the incarnation, but some believers have argued that it was the incarnation -- the uniting of the divine and the human in one person -- that was actually the most momentous event.
For myself, I don't think it's possible to pick just one event as
the most significant, but I think a short list of the most significant events might include:
1) The invention of agriculture
2) The invention of the wheel
3) The development of writing
4) The development of Vedic religion
5) The establishment of the Buddhist Sangha
6) The Edict of Milan
5) The establishment of the Caliphate under Abu Bakr
6) Various Muslim and Christian conquests
7) The invention of printing
8) The Enlightenment
9) The invention of motorized travel
10) Various discoveries in the area of microbiology
11) The taming of electricity
12) The invention of nuclear technology and weapons
13) The mastery of extraterrestrial travel, which is in its infancy and hasn't yet had the kind of effects it might eventually have, but seems momentous enough in its own right to be listed
I would have listed the taming of fire and the invention of speech, but I'm not convinced those accomplishments didn't precede our evolution as a species.