Traditionally, Buddha’s Birthday is known as Vesak or Visakah Puja (Buddha’s Birthday Celebrations). Vesak is the major Buddhist festival of the year as it celebrates the birth, enlightenment and death of the Buddha on the one day, the first full moon day in May, except in a leap year when the festival is held in June. This celebration is called Vesak being the name of the month in the Indian calendar. In Japan, which adopted the Gregorian calendar in the 19th century, Buddha’s Birthday always falls on April 8.
How Japanese people celebrate Buddha’s Birthday~
Buddhism in Japan~ http://asiasociety.org/education/buddhism-japan
Japanese Buddhist Art~
The Eight Auspicious Symbols of Buddhism~ http://www.exoticindiaart.com/article/symbols/