Alongside the Comic Market, the M3 is probably the second biggest even held in Japan when it comes to doujin music. Though overseas its commonly shortened as M3-xx (i.e., M3-38) with the “xx” being the number of the event (just like it happens with Comiket), in Japan it’s commonly known as “M3-year-spring/autumn” –i.e., M3-2016 Spring.
Held twice a year at Tokyo Ryuutsuu Center (TRC), once in spring, once in autumn, it’s condensed in just one day and its main focus is on music, video and multimedia. Though it’s not rare to see derivate works, its core are original works and vocaloid works.
Contrary to other events, as stated on the official site, M3 is an event designed on works which has connection with music –this mean, you will not find a manga or illustrations section, unless they are accompanied by a music CD, but you can and will find music works based upon manga, games, anime and so on.
As of today, December the 10th, 2016, the most recent M3, the M3-38, was held the 30th of October, 2016, while the next one, the M3-39, will be held the 30th of April, 2017.