I'm going out on a limb here and could be way (WAY) off base, but I would think that the Pirate Night would still be on whatever night it would normally be for a 7-night (is that what you're doing or are you doing the 3-night double dip?). So, if Castaway Cay is normally the final stop, then it would make sense that it would be that night.
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Disneyland - Dec 2011 Heather
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My Previous Trips DLR 1976, DLR 1981, DLR 1985, WDW 1988, DLR 1992, DLR 1993, WDW 1995 All-Star Music, WDW 1998 (Honeymoon) All-Star Sports, DLR 2000-2010 annually, WDW 2007 Animal Kingdom Lodge, DCL Wonder 2007, WDW 2009 Pop Century, DCL Magic 2009, WDW 2010 Port Orleans French Quarter