I guess it depends on the events... Historically, Rendezvous were held in the summer time. Aside from the monthly shoots, Rondy, and State Shoot that I do, I'll leave the other events for those who take part in them, to comment.
On the other side of the coin bringing us up to date. I think most folks vacation in the summer when school is out. I expect this may have the most bearing on when events are held?
I do agree, I would rather do Rendezvous in the early Spring or in the Fall, though in my area I don't see that happening anytime soon, if at all...