I Skyped Maltz. He said he has been asked before about "Merry Christmas" and "Happy New Year." He thought they were silly things to say (and, of course, there are no traditional Klingon ways to say these things), but he said that if you have to say "Happy New Year," you could say DIS chu' yItIv or DIS chu' DatIvjaj or DIS chu' botIvjaj or the like.
For "Merry Christmas," he wasn't so sure. He said that perhaps you could use the name of the holiday (which would be QISmaS) and say QISmaS yItIv or QISmaS DatIvjaj and so on. He thought that people who know about Christmas would understand that. He also liked my sister-in-law's suggestion: wornagh yItlhutlh!.