An official anime site detailing the content of the 2-hour Pokémon movie special to air July 17th has been put up. In addition to a TV airing of last year's movie, it will also have an all-new TV special starring Diancie. Translations follow:

A celebration of Pokémon the Movie XY's premiere!!
"Extremespeed Genesect: The Awakening of Mewtwo" and world exclusive footage of Diancie will be shown together!!

All of the above will air together in a single amazing special Thursday July 17th from 7:00pm to 8:54pm!
The program will be hosted by Mayu "Mayuyu" Watanabe from AKB48

Story

"Diancie, Princess of the Ore Country" is a story set before Diancie, the mythical Pokémon serving as the protagonist in this year's movie "Pokémon the Movie XY: The Cocoon of Destruction and Diancie", met Satoshi.

Diancie, the Diamond Ore Country's princess, and her Melecie butlers set off on a journey searching for the legendary Pokémon Xerneas in order to save the Ore Country from disaster. What awaits them on this journey!?

Staff and Cast

Executive Director: Kunihiko Yuyama
Screenplay: Junki Takegami
Storyboard/Assistant Director: Kentaro Yamaguchi
Production: Pikachu Project

Diancie: Marika Matsumoto
Majima: Kensuke Sato
Narration: Unsho Ishizuka

Source: http://www.tv-tokyo.co.jp/anime/diancie/index.html