I was looking at an xmas cruise xmas week, on carnival and the prices were half the price of new years, infact they were about the same as a spring or fall cruise. I think that the balcony started at around 899. pp.
now new years and thanksgiving...thats another animal by itself.
I think xmas is a time to be with family, so many hesitate to cruise. Me on the other hand would love to do this, but will most likely wait till my daughter is married and my son is in highschool (tho as a nurse i never get anything like a week off at xmas!)