I posted this last year, so indulge me in doing a repeat:
This week is Obon, a holiday where you are supposed to honor your ancestors, typically at their family shrine. However, the way most celebrate today is that you return to your family home (that is, you leave the Big City to return to grandma and grandpa's house.) In that way, it becomes a bit like American Thanksgiving. And that's especially true in the sense that this is Japan's biggest traveling week. Hotels are booked, flights and bullet trains are filled, but many businesses (from corporations to mom-and-pops') are closed.
It's a great time to visit Tokyo, because instead of thousands of people crowding the streets, you only got hundreds.
That, and the week ends with a giant festival and fireworks display over the river in my neighborhood. Yay, ancestors!
This year is no different. In fact, I estimate that if even 0.1% of the city population has fled for vacations and/or for visiting families, then that means about 35,000 people aren't around right now. That's a significant decrease in street AND pedestrian traffic! The down side is that many things are closed as well. For example, my favorite destination whenever I return to Tokyo after a trip is to stop at the Ootoya restaurant on the way home. Couldn't do that this time, as the business center in which it is located was shut down! Just goes to show, that with every silver lining, there is a dark cloud.