This year, mids had to be back from winter break on Jan. 4 (a Thursday). They were able to leave after their last exam, which varied from roughly Dec. 15-19 depending on what courses they were taking. They likely won't know their departure date until some time in November when the exam schedule is released but will know the last possible date as soon as the 2018 fall academic calendar is released (of course, pending duty, sports, etc.).
If you want to do a cruise, spring break may be an option. At least one cruise line operates (or did in the past) out of Baltimore (very convenient) and the last couple of years had a cruise that coincided exactly with USNA's spring break. Mids in good academic status typically will have that entire time unless they are involved in a sport that trains over the break -- and they will obviously know this well in advance.
They should also know their summer plans sometime in the spring and, once set, those dates are pretty firm. Depending on what they decide to do and thus the amount and timing of their leave, there may be some time then.
I don't see a T-giving cruise working . . . ever . . . while at USNA. Sorry.