January 2013- Pokémon Direct aired- reveals Chespin, Fennekin, Froakie, Xerneas and Yveltal, along with the first gameplay footage.
Youtube Link.
February 2013- Coro Coro reveals Sylveon, pointedly keeping its type a secret.
Youtube Link
April 2013- Mega Mewtwo Y revealed, with almost no information beyond the design.
Youtube Link
May 2013- The publicity starts in earnest. Fletchling, Helioptile, Gogoat and Pancham are revealed, along with trainer customisation, riding Pokémon, Lumiose City etc.
Youtube Link.
June 2013- X and Y featured in the E3 Direct. Noivern and Vivillon are revealed, along with Fairy-type and Pokémon-Amie.
Youtube Link
June 2013- Mere hours after the Direct, a developer roundtable is held at E3, revealing Clauncher, Skrelp and Talonflame, along with Sky Battles and Horde Battles.
Youtube Link
June 2013 (again)- Coro Coro comes out and reveals Litleo, Flabébé, Scatterbug and Spewpa, along with Viola, Shauna, Tierno and Trevor, and confirming rollerskating.
Youtube Link
July 2013- Masuda sends a message to Japan Expo, revealing Honedge.
Youtube Link
July 2013- Coro Coro reveals Inkay, Malamar, Pangoro, Spritzee and Swirlix, along with Professor Sycamore, Team Flare, Clemont and Grant. The magazine promises "the scoop of the century" for August.
Youtube Link
July 2013- The Genesect movie releases in Japanese theatres, along with a trailer featuring Trevenant.
August 2013- Coro Coro reveals its big scoop- Mega Evolution. The concept is introduced, and Mega Lucario, Mega Blaziken, Mega Ampharos, Mega Mawile and Mega Absol are revealed, alongside a confirmation that Mega Mewtwo Y is a Mega. In addition, Skiddo, Dedenne and Bunnelby are revealed, along with Korrina and the existence of Super Training.
Youtube Link 1 Youtube Link 2
August 2013- At the World Championship opening ceremony, Masuda reveals Mega Kangaskhan.
September 2013- A Pokémon Direct airs, revealing Mega Venusaur, Mega Charizard Y and Mega Blastoise, as well as Pokémon Bank.
Youtube Link
September 2013- Coro Coro reveals Quilladin, Braixen, Frogadier, Tyrunt, Amaura, Furfrou, Meowstic, Mega Garchomp and Mega Mewtwo X, along with Diantha, the Team Flare Scientists and the reorganised Type Chart.
Youtube Link IGN also reveals Pyroar shortly afterwards.
September 2013- Jeux Video and IGN reveal Aurorus and Tyrantrum, respectively.
September 2013- The Pokémon Facebook page reveals Doublade.
October 2013- Pokémon Origins airs, bringing with it the reveal of Mega Charizard X.
Youtube Link
October 2013- A Japanese Pamphlet reveals Aromatisse and Slurpuff.
That's basically the lot. Coro Coro revealed the vast majority of the Pokémon and Megas in its October edition, but that came out after the games launched.
Man, June 2013 was a crazy month. Three different sets of reveals, all basically happening at the same time.
I'm actually inclined to think they revealed too much in the runup to X and Y, mind- certainly, with the reduced amount of new Pokémon they could have stood to keep a few of them back.