Late May
This event is seasonal and so dates may fluctuate each year.
During the first half of June, Mizumoto Park in Katsushika ward comes alive with over 14,000 iris flowers of 100 varieties. The park is open all day and entrance is free. It is just under an hour on the train from Tokyo Station. The festival also takes place at Horikiri Shobuen Iris Garden a little closer to Tokyo (Google Maps link). Here 6,000 flowers of 200 varieties can be seen.