From what I know, they don't necessarily "open up", as the need for a handicapped room can arise the day before or the day of the cruise if someone has an accident. Empty rooms are sometimes assigned to guests who book a guarantee cabin, and also they can be moved from that cabin (before the ship sails) on a moments notice if someone suddenly has a need for a handicapped room. Also sometimes normal guests are able to book those cabins if thats all that remains in that category. They may even leave those cabins empty, it all depends.