According to Serebii:
In The Games Department
Pokémon Sun & Moon
A new event has begun around the globe. This event gives the Mega Stones for Mewtwo, the Mewtwonite X and the Mewtwonite Y. This is given with the code: M2DESCENT. Currently there is no end date known for this distribution so be sure to get it while you still can.
Edit @ 09:54: It is also confirmed that the gift for the Beedrillite and Mawilite for entry in the 2017 International Challenge February will begin on March 7th 2017. A shared serial code to give both of these stones will also be available from April.