From the ticketmaster website:

The Killers show, scheduled for the WaMu Theater on Sunday, May 20 has been rescheduled for Sunday, May 27. Due to his on-going recovery from bronchitis, Killers lead singer Brandon Flowers has been ordered by his personal physician to rest his voice for another day.

The event's promoter, AEG Live, said that all tickets purchased for the May 20th performance will be honored On May 27th.