Today and tomorrow, the annual Sanja-Matsuri (三社祭) is being held in Asakusa. For a very good outline of what it’s about and what happens each day, check out the Wikipedia page. You can also visit the official site (the English page is not really worth visiting) and the schedule (in Japanese) is here.
It is one of the “Big 3” Shinto festivals in Tokyo (Kanda Matsuri and Sannō Matsuri are the other two), so it is an amazing spectacle with a lot of great photo opportunities. It is also a GREAT place to get some candid street portraits, so if you are looking for something to do tomorrow, check it out.
Here are a few of the candid street portraits I took last year:
To get to the festival, just take the train to Asakusa and exit out onto Kaminarimon Dori (about a 2 minute walk from Asakusa Station on the Tokyo Metro Ginza Line).