There's Boss Event, daily, at ~23:30 Server Time. (you can check server's time @KO Panel)
And, for no misunderstandings, make sure to find a clan or at least a party for the event, 'cause there are only few bosses that you can solo (i.e. cavalry, mini bach/duke/bishop, bach/duke/bishop, trap, vice-commandant Guard, attila). Rest requires a party.