Definately the most visited event that will be held in Prague and Ostrava is IIHF Hockey World Championship. Event takes plase from 1st of May till 17th of May.
More info can be found at

WEB
www.iihfworlds2015.com

FACEBOOK
http://www.facebook.com/MSIIHF2015
http://www.facebook.com/2015IIHFWorlds

TWITTER
http://twitter.com/MSIIHF2015
http://twitter.com/2015IIHFWorlds