As one of the larger and wilder festivals in Tokyo, the Sanja Matsuri attracts massive crowds over the 3 day period. I went on a Sunday and it was very crowded, to the extent of being almost claustrophobic. The matsuri is a Shinto event, but it is Senso-ji, a temple, where everyone gathers and the famed matsuri stalls are located. The smaller Asakusa Shrine is actually located near the temple but I was unable to reach it due to human congestion. If you dislike crowds, stay on the fringes of the road. An unforgettable event not to be missed if you are around for it!