Passage: To celebrate the 50th anniversary of the show, a special 3D episode, "The Day of the Doctor", was broadcast in 2013. In March 2013, it was announced that Tennant and Piper would be returning, and that the episode would have a limited cinematic release worldwide.In April 2015, Steven Moffat confirmed that Doctor Who would run for at least another five years, extending the show until 2020. In June 2017, it was announced that due to the terms of a deal between BBC Worldwide and SMG Pictures in China, the company has first right of refusal on the purchase for the Chinese market of future series of the programme until and including Series 15.

Question: what was broadccast in 2013 to celebrate a 50 year anniversary?
The Day of the Doctor