The dates vary slightly year to year, due to fluctuations in the calendar. But for the last decade or so the Haunted Mansion Holiday rehab calendar has looked like this.
The installation takes 13 to 15 days, and usually begins the fourth Monday in August, usually reopening on the Friday after Labor Day. Sometimes they soft open on Thursday afternoon.
The removal takes 9 to 11 days and usually begins the second Monday in January.
7 or 8 years ago the removal rehab was extended into mid February due to a full repaint and wood refurbishment on the Mansion building itself.