What is a "non event"?

Not sure, but I'd think it would refer to something that was supposed to be a big deal when it occurs, but then totally fails to amount to much of anything. Like a planned "massive" protest that ends up with only 5 people attending?

It is an event that doesn't measure up to its hype. Y2K was a nonevent. All the hype and the fear mongering etc etc etc. The clock ticked over to the new year and nothing happened. All the fear and anxiety was for naught.

A thing that happens with no obvious reaction, notoriety or repercussion. When no one cares that the even took place.

Something that does not live up to the description attributed to it

It is like when Joe Biden opens his mouth, totally uneventful.