According to the legend, a statue of the Kannon (deity for mercy) was found in the river by two fishermen in the year of 628. The chief of their village recognized the sanctity of the statue and rebuilt his own house into a temple for the villagers to worship Kannon. This is how Senso-ji was founded, one of the most famous temples in Tokyo and an Icon of Japan often featured on books and movies.
Every year, the 3rd weekend of May, Senso-ji honors these 3 founders of the temple (Hinokuma Hamanari, Hinokuma Takenari and Hajino Nakamoto) with the largest and wildest festival in Tokyo. During this 3 days celebration they have multiple activities and performances, where the highlight is the parade of the 3 big mikoshis (portable shrines) carried around the streets by different groups of people. This is also a unique opportunity to see Yakusa (Japanese mafia) members, as they traditionally join the festival to carry their Mikoshi show their very distinctive tattoos.
Thousands of people attend to this festival, it gets so crowded that it is difficult to move around, especially around the main Temple, but still very entertaining and a must see festival if you are near Tokyo.
Full details (When, where, how and more) can be found here: http://discoverjapan.info/Details/AsakusaSanjaMatsuri
The following are some pictures I took of the event