The dates for IllumiNations being down are June 5th - September 26th. Seems that this time will be used to rehab IllumiNations ready for it's 2000 replacement. The new show is an extensive rework, involving new hardware. The 3 month downtime maybe confirms the rumour that new lasers will be installed in American Adventure, Mexico and Canada.
An opening date just a few days ahead of the October millennium launch gives a few days of soft opening for the new show.