A relic is paraded to Tu Dam Pagoda in Vietnam
Buddha's Birthday (known by different names in different countries) is celebrated on the 8th day of the 4th month in the Chinese Lunar Calendar, primarily by Mahayana Buddhists.
Except in Japan. Because Japan formally adopted the Gregorian calendar, Buddah's Birthday is celebrated there on April 8th, and sometimes May 8th, but hardly ever on the "orthodox" Chinese Lunar Calendar date. It's like the U.S. with our standard system of measurements, refusing to use metric.
The Japanese Holiday is also celebrated a little differently, typically with flower festivals at various shrines, most notably Asuka-dera. Small mikoshi are decorated with flowers and believers pour ama-chan (made from a type of hydrangea) over small statues "as if bathing a newborn child."
Anyway, this year Buddha's Birthday (which is really fun to say, try it) fell on the 16th or 17th-- depending on whom you ask. Millions of devtoees in Nepal, India, Vietnam, Sri Lanka, Thailand and others celebrated by chanting sutras and eating special dishes.
Happy Birthday Buddha, you bishi!
A dragon lantern in Seoul's annual festival