Fireflies are hard to find in metropolitan Tokyo as they can only live around very clean bodies of water. In recent years, the city has made much effort to revive the population of fireflies by improving the natural environment. In Yokohama, Sankei-en Garden is a great place to see them. Check the website for the exact dates, as the viewing usually lasts for about a week in early June.
Find out more about Sankeien Garden.