Each year in June, Tokyo's Hakusan Shrine is home to an impressive ajisai (hydrangea) festival, where around 3000 blooms in various shades of purple and blue decorate the shrine and the neighboring Hakusan Park. Hydrangeas are one of the highlights of the rainy season in Japan, and they add a welcome dose of color to this time of year.
Do note that the festival dates are subject to change depending on when the hydrangeas are blooming - as a rough indication, previous years have been around the second and third weeks of June. It's wise to check the official Bunkyo municipal website before attending to ensure the flowers will be at their best.
Admission to the event is free of charge.