I've just come back from a Twixmas short break. It was quite cheap, as the hotel needed to fill rooms in that annoying lull between Christmas parties and NYE. So YABU as Twixmas meant a cheap relaxing break with DH and the DC to get over having spent Christmas with both sets of parents.
YANBU to think it is a pretty annoying marketing term though. And it does make me think of Selection Boxes.