For the spring 2012 addition of the festival, Panasonic offered to spice things up by adding 100,000 lights to the Sumida River in Tokyo. The lights represent the fireflies that were once found along the river. The festival has its roots in Edo-era traditions of catching fireflies by the Sumida in spring.
The lights are powered by solar and were collected in a large net before reaching the sea. They were specially designed by Panasonic for the festival.
The event was a veiled promotion for Tokyo Sky Tree that's set to open May 22, 2012. Tokyo Sky Tree showed off it's lights for the festival which corresponded with a full moon and golden week (May 6th).
It's unclear if the 2012 event will make something of the (previously) little known Hotaru Matsuri. The Sumida river area is home to many of Tokyo's largest festivals including the wild Sanja Matsuri and Asakusa Samba Carnival and the massive Sumida River Fireworks.