In 2007 the Japanese Parliament reestablished the 29th of April as a day to celebrate the birth of Emperor Shōwa (Emperor Hirohito, ruled during World War II). Until his death in 1989 the day was seen as a day to celebrate the birth of the Emperor. After his death the name was changed to Greenery Day to distance the day from the war time leader but still connect the day to him as he was known for having a deep interest in nature.
Several attempts where made to change the name back before it was successful but due to the devastating events of the war brought on the neighbor of Japan there was great opposition from both outside and inside the country. In 2005 by an overwhelming majority (202-14) the Parliament of Japan changed the name and moved Greenery Day to May 4th.
There is still a lot of opposition from many groups to this name change but from what I have been able to gather it does not seem that the day is actually celebrated but used to signal the start of Golden Week in Japan, one of the vacation times in the country.