April 8 Flower Festival Celebrating Buddha's Birthday
Buddha's birthday is celebrated on April 8 in Japan
April 8 is not a national holiday in Japan, but there are festivals at temples everywhere in the country. This day is the celebration of Buddha's birthday, often called Flower Festival （花まつり） or Kanbutsu-e （潅仏会）.
In Japan the date for Buddha's Birthday is April 8, but some temples start the celebrations earlier. The festival is different at each temple.
Celebrating the Flower Festival
I have watched the parade with many small children participating arrive at Senso-ji in Asakusa and was surprised to see an elephant at the temple.
"According to legend, the Buddha’s mother (Lady Māyā, aka Mahāmāyā or Māyādevī) dreamt of a divine being atop a white elephant descending from heaven, touching her side, and entering her womb. The elephant imagery indicates that her child came from the pure land known as Tuṣita, home of Bodhisattva Maitreya (Jp. = Miroku). The child was born near Kapilavatthu (in current-day Nepal), emerging from his mother's side, which emitted a seven-colored light that brought forth the infant, who then took seven steps forward while pointing his right hand to the heavens and left hand to earth (see image here), saying: "I alone am honored in heaven and on earth (tenjō tenga yuiga dokuson 天上天下唯我独尊)."...His mother died seven days after giving birth."
Full quote and a lot more information here http://www.onmarkproductions.com/html/shaka.shtml
Overview of the festival in English