One might say the Original Event in mathematics is Euclids axiomatisation of Plane Geometry.
Subsequent axiomatisation - such as Peanos Axioms for the natural numbers being an echo of that Event, and so not an Event as such, in itself. But merely an application of an already discovered idea - axiomatisation.
What are the nature of subsequent Events in mathematics?
One might pause here, and say why, given the natural numbers, did axiomatisation wait two millenia.
It surely isn't dependent on the difficulty of the task.
So, Peanos axioms is possibly then a sign of an Event.