I think the date depends on whether sakura comes into bloom or not. You should go when sakura is in full bloom.
This picture shows that sakura in Tokyo bloomed on March 25 last year.
I've found this Sakurabashi Hana-Matsuri at Enjoy Tokyo.
This page says that the festival is on April 5 and 6 at Sumida Park. The nearest station is Asakusa.
http://www.enjoytokyo.jp/OD004Detail...VENT_ID=122194
Rurubu gave me three different festivals named Sakura Matsuri from late March to early April. Which one?
Ueno Sakura Matsuri (JR Ueno)
http://www.rurubu.com/event/detail.asp?ID=11357
Bokutei Sakura Matsuri (Subway Honjo-Azumabashi)
http://www.rurubu.com/event/detail.asp?ID=11344
Chiyoda no Sakura Matsuri from March 28 to April 6 (Subway Kudanshita)
http://www.rurubu.com/event/detail.asp?ID=11364