AIR, The TV-Summer Special
This fits into the events of the Spring TV 12(13) episode series. It is a more detailed look into the arc of 1000 summers ago, since this was mostly glossed over and unclear in the TV series. kawaii desu!
Also check out:
AIR, The TV Series - Same Production, Main Story (Kyoani)
AIR, The Movie Main Story (Different Production)
The Melancholy of Suzumiya Haruhi Also done by Kyoani (not a Key story).