Nazo no Kanojo X: Nazo no Natsu Matsuri
English name: Mysterious Girlfriend X: A Mysterious Summer Festival
Genres: Mystery, Romance, Ecchi
Type: OVA, Episodes: 1
Aired: Aug 23, 2012
Studio: Hoods Entertainment
Source: Manga
Rating: PG-13 - Teens 13 or older
Synopsis:
OVA released with the 9th volume of the manga. 

The day when Tsubaki goes to the summer festival with Ueno, Oka, and Urabe.