Board Thread:The Wikia Archives/@comment-216.165.251.187-20150920164837/@comment-1398253-20150924172725

That would logically be Series 8, because it aired around six weeks after the rest of that season. However, the production codes listed on the articles are 4.12 for Death in Heaven and 5.X for Last Christmas. Which wouldn't place them in the same season (though I must say, those production codes don't make sense, since The Stolen Earth is also production code 4.12).