Episode 19

An Ordinary Day (なんでもない日) is the 19th episode of the Ao no Exorcist anime overall and of the first season. It first aired on August 21, 2011.

Synopsis
The Exwires learn that Izumo's birthday is coming and they decide to make a surprise party to her. Kamiki becomes supicious of the other students' activity, especially when she sees Shima with Paku. Despite all their efforts to make the party a secret, Kamiki ends up figuring the truth out and claims that it should also be dedicated to those whose birthday already passed too. In the end, the party becomes a celebration to everyone's friendship, but their happiness is cut short when Yukio comes with news that the monastery where he and Rin were raised was attacked.

Trivia

 * Due to celebrating their birthday and Christmas on the same day for 15 years, Rin and Yukio had no idea that a Christmas cake and birthday cake looked different.
 * Rin's cooking skills are shown to be good enough to impress even Mephisto (who was given the X-mas cake Rin made by accident as a gift from the students.)

Manga & Anime Differences
T his is an anime only  episode, it does not appear in the  Manga, but is very similar to Chapter 88.