Given the title of your thread, it should be noted that J.K. hasn't approved any Christmas decoration inside the Wizarding World of Harry Potter. That of course could change at any time. The bulk of USH decorations are in the front of the park in the New York/Production Central sections. At IOA, they are mainly in Port of Entry and Seuss Landing.
Having been to Harry Potter during the Holiday seasons, the Christmas decorations are very very subtle. I only saw them at Hogsmeade not at Diagon Alley, this may change in the future. At Hogsmeade (IOA) they had a few wreaths and garlands. No trees all very understated. I did like the wreaths on Honeydukes and look of little pieces of candy on them. But Diagon Alley was not really decorated.
The rest of the park is nicely decorated. I was impressed. They even matched the trees around Spiderman to his Colors. But Suess Landing is festive and impressive as is the New York area.
There is nothing announced, but there have been surveys and rumors suggesting that there will be new things this Christmas for the parks, including possibly a "Yule Ball" type event for the Harry Potter areas.