Cherry Blossom season in Japan is in Spring during the month of April generally (although it varies across the country). I celebrated the coming of Spring, Japanese style, on 15 April 2001 at a hanami (flower viewing) party in Ojoji, Nagano.
To celebrate Hanami you should get a group of people together and bring along a BBQ and some alcohol. Next you have to find a cherry tree that is unoccupied and set up. There are popular Hanami spots all over Japan but any Cherry Tree will do the trick! You should then proceed to get very drunk and have a lot of fun talking and laughing with the locals!