The concert will be held on the 3rd of September (3/9/2012). 3/9 in Japanese can be pronounced as Gu (9) Mi(3) and this has been acknowledged by fans as GUMI's day.
Using Nawota's SYNCLOiD system and Amid-P's amid screens, this will be a power up of previous GUMI's concert in Nicofarre.
Here are the demonstration videos uploaded by Nawota.
This is the concert demo footage.
And here's the demo footage of SYNCLOiD.
The event is of free entrance and a NicoLive broadcast will be arranged on that day.
The official twitter for Miku no Hi DaiKanshasai : @GUMIsDay
Source : http://vocaloid.blog120.fc2.com/blog-entry-12716.html