If there's one thing more spectacular than the Sakura (Cherry Blossoms) in full bloom, it is seeing them lit up at night - "Yozakura" in Japanese. Ueno Park is one of the most popular spots in Tokyo for Hanami (Cherry Blossom Viewing or Picnic-ing), and is buzzing each night of the season with thousands of Hanami picnic partiers.
Illumination is generally from the end of March to around the first two weeks of April, to co-incide with the height of the short-lived Sakura blossoming. Dates vary from year to year. These are a few of the pictures I took April 12th last year at Ueno Park. Illumination is from 5:30 until 10pm.
Other top spots in Tokyo with Sakura Illuination include Chidorigafuchi (part of the moat around Chiyoda, the location of the Imperial Palace) and Sumida Park (along the Sumida River). That's where I'm planning for my Yozakura outing this year. (very excited)
I thoroughly enjoyed the beautiful Yozakura as I walked through Ueno Park, but unlike places such as Shinjuku Gyoen, (see link for last year's photos) Ueno Park is not the most family-friendly Hanami picnic spot as it is quite crowded, and the large majority of picnic-ers are consuming (substantial amounts of) beer. They are otherwise well-behaved, however, and I did see one or two other young families like ourselves enjoying the evening.
The end of March to the first half of April is definitely a good time to visit Japan.
Happy Hanami 2013!