If you want to watch it with subtitles, go April 29; the English-dubbed version airs April 30. The original storyline is not adapted from the manga but tells a new story. From the press release:
The story begins as L takes on the “Kira” case, in which countless criminals are dying under mysterious circumstances. L successfully solves the case and stops Kira, but only after sacrificing his life, leaving himself with only 23 days left to live. For his final case L faces down a bio-terrorist group trying to wipe out humanity with a deadly virus ten times more potent than Ebola. As L tries to formulate an antidote with a scientist, he must also save the lives of two children who have no one else to turn to.
Tickets are available through Fathom Events. Here are theaters for subtitles and dubbing. In the second version, Alessandro Juliani voices L; he also provided the voice in the anime series. The film is unrated but is considered to be equivalent to an R.