Ermmm :S m not quite sure
but they do regularly update the site if that's what you mean with stuff that will be appearing at the expos aswell as future dates after one has just finished
And also when you go to one, they give you the dates for the next ones in the show guide they give you when you buy your ticket.
The site for the one in london is :
LONDONEXPO.COM - London MCM Expo The stars you want to meet & attractions you want to see!