Sumidagawa paper lantern floating (灯篭流し, tōrō-nagashi) festival began in 1946 to pay respects and mourn those who lost their lives in the air raids and fires of the war. Today the event attracts many people to the river banks of the Sumida River, and is popular not just with Japanese but with foreign visitors, too.
Many, of course, come just to watch the lanterns floating in the evening on the river, but you can also purchase and set free a lantern of your own for ¥1,500. The area for registration is at the bottom of Sumida Garden nearest Asakusa Station between Azumabashi Bridge and Kototoi Bridge on the west side of the river (Google Maps link). Registration is open from 2pm and ends when they have run out of lanterns.