How do you get Diancie in Pokemon X and Y?

Diancie is an event-only Pokémon so it cannot be found in Pokémon X or Pokémon Y, you can only get it through special Nintendo events.

In the United States, you will be able to receive a Diancie serial code between October 27, 2014 and November 16, 2014 by going to a GameStop store. In Europe, the distribution date is between 6 November, 2014 and 28 November, 2014, in various retailing stores depending on where you live. Japanese people that bought tickets to the movie Diancie and the Cocoon of Destruction also received a Diancie.