I don’t know about occurring only once but I would say an ice age is a pretty rare event and a pretty big one. I’m not sure, however, what state of evolution ‘people’ were in at the time so as to count as ‘witnessed by a lot of people’. You also might consider a meteor strike. While meteor showers are common enough meteors actually impacting the earth much less being witnessed are pretty rare (although they too have happened more than once).
Beyond that I’d vote for a tidal wave. While they’ve certainly occurred more than once they are pretty rare events to witness (think of all the video you’ve seen of tornados and think of the number showing a tidal wave).
As for a rare cosmic event that can be witnessed by a lot of people I’d say a supernova. Some have been visible to people without benefit of a telescope. However, some events would be even rarer such as the formation of a black hole or the collision of two galaxies. A black hole has never been viewed being formed and a collision of galaxies takes place across a huge timescale compared to a human lifetime.
A once in a lifetime cosmic event viewed by many people might be the impact of Shoemaker/Levy-9 into Jupiter.