Hello, I've noticed quite a few posts about this tour.
This is a lovely trip, I was the TM for it last year and the year before.
The hotels are really nice and well-located, with good atmospheres for both celebrations. The excursions are interesting and there's enough to keep you busy, as well as giving you time to relax, enjoy the two places and experience different festive traditions.
The Dolomites area is a stunning part of Italy, and you may even get a bit of snow on Christmas Day?
Croatia is beautiful and at that time of year the weather is usually a bit milder than in the Dolomites, so you may get a bit of winter sun to warm you up before going home!
Make sure you pack a nice outfit for the Christmas Day and NY Eve celebrations, as well as some warm shoes, gloves, etc.., as it could be quite cold in the Dols.
Then be ready to have fun on a really good festive holiday.