He told the New York Times that 'A Christmas Carol' would balance out the "dark" nature of 'The End of Time'.
"There had been a feeling that we needed a really, really Christmassy one, because the previous [Christmas special] had been David Tennant's exit from the role," he explained. "It had been quite dark and not as Christmassy as normal. We had wanted to reset it and make [this year's episode] a hugely jolly, merry one. Well, jolly, merry and a little bit sad."
Moffat also praised the original Charles Dickens novella A Christmas Carol, which serves as the festive special's inspiration.
"It's a fantastic story," he said. "Time travel at Christmas says A Christmas Carol and it says Doctor Who. The moment you think of it, if you have The Doctor as the Ghost of Christmas Past, it sort of makes sense."
'A Christmas Carol' will air on Christmas Day on BBC One in the UK and on BBC America in the US.