Greenery Day or Midori no Hi is celebrated today, May 4, in Japan. (Midori means green and Hi means day.) The holiday honors former Emperor Showa who "loved plants and nature" (he died in 1989). The original celebration used to take place on the emperor's birthday, April 29.
According to japan-guide.com, "Before being declared Greenery Day, May 4 used to be a national holiday due to a law, which declares a day, that falls between two national holidays, a national holiday." Greenery Day is part of Golden Week, a week of holidays, in Japan.
Greenery Day comes on the heels of Earth Day and Arbor Day, both celebrated in April.