According to Anime News Network, there was announced an orchestral concert by a 70 amateur musicians orchestra under the name “Walpurgis Night/Die Walpurgisnacht Symphoniker”.
The orchestra has received consent for the concert from Sony, Nitro+ as well as JASRAC and will be conducted by Kenichi Shimura. A total of 15 Yuki Kajiura musics from the TV anime (including 5 character themes), rearranged for orchestra are expected to be performed.
Artist: “Walpurgis Night/Die Walpurgisnacht Symphoniker”
Concert: Walpurgis no Yoru no Yume
Date: April 6, 2013
Venue: Ichō Hall at the Hachioji City Art & Cultural Hall in Tokyo
Tickets: FREE (sold on concert date starting at 10:00 a.m.)
Start: 5:30 p.m.
Orchestra website
Thanks to chrisgarci for the tip!