[DTV]

Before 2001, Christmas idents were retired at closedown on Boxing Day or on a proceeding Bank Holiday, but the latter scenario wasn't always the case.  So, really, we should be grateful that Christmas idents start early December and end early New Year's Day.

I suspect part of the reason for the increased length of time that Christmas idents are on air is the increasing cost of idents. It'd be harder to justify some Christmas idents, particularly some of those during the circle era, if they were only up for a few days.