According to the twitter of Pony Canyon PR staff, the premiere date of K-ON! Movie
was announced to be December 3rd 2011. The announcement was made at K-ON! live event "Come with Me!" held on February 20th at Saitama Super Arena.
According to the tweet by anime log designer Kanaya Akemi, the director Yamada Naoko
said the movie will be a brandnew episode , not a recap. The story takes place before the graduation of Yui.
Sources: Nojitetsu's twitter
, Kanaya Akemi's twitter