Episode 13, included in the final volume of the BD/DVD.

Taking place directly after the events of "Chuunibyou demo Koi ga Shitai!", Christmas is coming up and Rikka is told by Nibutani that this is an opportunity for her to get closer to Yuuta. Under Makoto's suggestion, they plan a Christmas party at Sanae's Mansion.

[Source: MyAnimeList]