There are a lot of different festivals In Japan and this page covers just a few of the more famous ones.
Cherry Blossom Festival
Here is a flash slideshow explaining more about cherry blossom festival ( or Sakura Matsuri):
Hinamatsuri (雛祭り) or Doll festival as it is known in english is a day to celebrate dolls. This day is also known as Girl's Day as well because a lot of girls celebrate this day (due to the fact that girls tend to like dolls more than boys). This happens on March the 3rd every year. Here is some pictures from this festival:
Shichi-Go-San () is a festival for all children aged between 3, 5 and 7. This festival is usually held in november 15th and celebrates the rite of passage for three and seven year-old girls and five year-old boys. This celebrates the child moving on into middle childhood. Here are some images from this festival below: