Kanda Matsuri is one of three major Matsuri in Japan (Gion Matsuri in Kyoto, Tenjin Matsuri in Osaka and Kanda Matsuri in Tokyo), and also considered as one of three major Matsuri in Tokyo.
There is no record for the origin of this Matsuri, but it became popular Matsuri during Edo period. In Edo period, it was used to be Dashi, parade float, Matsuri, but it changed to Mikoshi Matsuri after Edo period was over.
Kanda Matsuri is held the nearest weekend of May 15. On May 15 there is traditional ceremony at Kanda Shrine. If 15 is Sunday, the ceremony moved to the other day.
It only happens every other years. Next one will be 2015.
3 Mikoshi floats of Kanda Shrine will be pulled by 300 people with traditional costumes and travel around Kanda, Akihabara, Nihonbashi, Otemachi and Marunouchi. Other Dashi, parade floats join on the way. This parade is held on Saturday.
All Mikoshi around the shrine will join the parade to Kanda Shrine. At the Kanda Shrine, Mikoshi and people receive the ritual reception from the priest. 100 thousands of people join this festival and it goes from morning to night. It happens on Sunday.
The main Matsuri is held in May 15. If May 15 was Sunday, it would move to other day.
There is traditional ceremony at the Shrine.
There is Wadaiko festival at the shrine, too.
Official Web: http://www.kandamyoujin.or.jp/kandamatsuri/