The Sanno Festival (山王祭) is the last of Tokyo’s major festivals and one of the three great festivals of Edo. It takes place over an 11-day period at Hie Shrine in Chiyoda on even-numbered years. There are many small events over the course of the festival, but the main attraction is the 300-strong procession through central Tokyo replete with floats and mikoshi (portable shrines) which departs from Hie Shrine at 7:30am and returns at 4:45pm on the 8th. Hie Shrine is in fact the designated protector of the city (it was built on high ground so that it could be seen from the former Edo Castle), and the procession goes via Tokyo Imperial Palace so that the head priest can perform a rituals to offer prayers to the emperor. See here for the full events schedule (Japanese only).
Event details are taken from official sources whenever possible. It is always prudent to check the official event website before attending to make sure that there have not been any changes. If you know of an error in the calendar, then please let me know through the contact form.