Not too long ago, an edit was added to the tournament schedule page stating that tickets would need to be delayed for the World Championships. A new date has not yet been announced.
Editor's Note - Aug 15: Regretfully, there's been an unexpected delay in issuing tickets for the World Championship and sales will not begin at 12PM PDT today as previously announced. We're working diligently to resolve the problem and will provide an update when we've confirmed a new sale date and time.
Riot promised to let players know 48 hours in advance when they begin to sell tickets once again.