The Radio Times always covers a Saturday to Friday period, with the Christmas issue simply having two consecutive such periods in it.
Nothing has changed this year; it's just that Christmas Day being on a Saturday makes things a bit awkward for the publishers. If they included New Year's Day, they'd then get complaints from people who bought the following regular issue about the fact that they were only getting new programme listings for 6 days instead of 7 (because New Year's Day would be in both issues).