On the Western Cruise it's the night you are in Cozumel (Wednesday). Not sure about the Eastern Cruise, it may be the same Wednesday (St. Thomas/St John), someone please correct me if I am wrong.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!