A Christmas special is an episode of QI which has questions themed around Christmas, the western holiday based around the birth of Jesus. In these episodes, the set is redesigned, usually to add Christmas trees, tinsel, baubles and other Christmas-themed decorations. The title of the episode is also Christmas-themed.
The first Christmas special was the A Series episode "Advent", on 23 December 2003. The only series not to have a Christmas special is the C Series. The episodes are first broadcast on or around Christmas, and are sometimes bumped up to a more popular channel (e.g. "Advent" aired on BBC Two first, although other A Series episodes originally aired on BBC Four). The first Christmas special was a series finale, but this is not always the case.