The Christmas Costume Set is now available on the PlayStation Store for free on both the PlayStation 4 and PlayStation 3 versions of Persona 5. You can check out a preview image of the new outfits, as well as the product description, below (via the aforementioned PlayStation Store lisitngs):
A special present as thanks for playing Persona 5! Contains Christmas costumes for the Phantom Thieves.
Persona 5 was released for the PS4 and PS3 on the 4th of April in North America. If you are interested, the latest Persona 5 is available for purchase with this link on Amazon.