Matsuri, festivals, and bon odori guide?

I find it so difficult to know the dates and places of matsuri, summer festivals, or fireworks!
I often check Timeout Tokyo, Tokyo Cheapo, or Flip Japan, but I still miss many events. I work in Roppongi, and today I saw some workers installing dance installations in Roppongi Hills, but I can’t find any information about that! The same thing is happening near my place, where my local temple will have some celebrations.

Is there a reliable page, even in Japanese, that lists them?