By entering the code MATSUBUSA, you will be given 5 Mega Stones for your copy of Pokemon Sun or Moon. The stones include Sceptilite, Blazikenite, Swampertite, Banettite and Cameruptite. There is no indicator for how long the event will last, so it might be better to download sooner than later.
Update: The code seems to now be available in Europe too, so the article has been updated to reflect that.
