Pokemon Omega Ruby and Alpha Sapphire released last year as the most recent games in the mainline Pokemon franchises. The Pokemon Company has continued to support both it and X and Y, which is why the latter two received the mythical Diancie last year, and now those with Pokemon Omega Ruby and Alpha Sapphire can do the same.
The mythical Pokemon Diancie cannot be gotten in this game otherwise, so this is the time to get it. To start off, go to the menu before starting up the game and choose Mystery Gift.
On the next step, choose “Receive Gift” and then “Get Via Internet” to advance further. After a little time, the Mythical Pokemon Diancie will pop up and choose “Yes.”
Now, fire up your copy of Pokemon Omega Ruby or Alpha Sapphire and head to any Pokemon Center and make sure to have a spot open in your party.
After you get Diancie in your party, leave whichever Pokemon Center you are at and re-enter. Upon going in, you will see a scene that will end with you getting the Diancite that will allow you to turn it into Mega Diancie.
Make sure to do this right now, as Diancie will only be available in Pokemon Omega Ruby and Alpha Sapphire through the weekend.