Today marks the beginning of Golden Week, which means that the entire country is on vacation. That's right, you don't get standard vacation days if you're a working professional in Japan--you get Golden Week.
Golden Week is actually a series of national holidays all bunched up together. Showa Day, Constitution Day, Greenery Day, and Children's Day all happen in the first week of May, so someone just decided that that would officially be a golden time for travel and leisure.
During this week, Tokyo is empty, plane tickets are super expensive, the rest of the world sees a sudden influx of Japanese tourists, and Narita Airport is in a constant state of chaos.
So don't be surprised if you see a billion Japanese businessmen with yellow hats and tour bus badges at major tourist points in the city you live in.
And be nice to them, because this might be the only chance they get all year to get out of their business suits and let it all hang out.