This year, 2015, the schedule of the Golden Week Holidays will be Wednesday, April 29; Sunday, May 3; Monday, May 4; Tuesday, May 5; and Wednesday, May 6. That includes the the four official holidays (Showa Day, Constitution Memorial Day, Greenery Day, and Children's Day), plus an extra “filler” holiday.
The extra holiday is generated when one of the official holidays falls on a Sunday. This gives the Japanese workers the same amount of holidays each year.
We Americans made a similar adjustment to many holidays by moving them from their original, fixed, date to the closest Monday. This was called the Uniform Monday Holiday Act. This gave American workers more three-day weekends.
For most Japanese, this is the longest holiday period that they can get for the entire year. By using a minimum of vacation time to bridge between the Wednesday and Saturday, a Japanese worker can get over a week off to visit family.
Many employers encourage this, as can be seen by the number of businesses that shut down completely during this period.
What does this mean for the average Tokyo expat? Well, don't expect to get any banking done, as these are also bank holidays. If you want to travel outside of Tokyo, reservations will be hard to get, as well as being more expensive than other times of the year.
However, the important things, restaurants and bars, will still be open for business.