Originally, the holiday was observed on 29 April, the birthday of Emperor Hirohito. In the Japanese system, the emperor's birthday is a national holiday (much as in the UK, though owing to notoriously fickle weather, the monarch's birthday is always celebrated on the first or second Saturday in June--no matter the actual birthday of the monarch--to increase the probability of pleasant weather).
Hirohito, or the Emperor Showa, reigned from 1926 until his death in 1989. I suppose after so many decades of observance, the holiday became entrenched in the culture. My interpretation may be correct, because after Hirohito's death the Japanese Government wished to continue observance (who wouldn't want another day off from work?) and so the holiday was moved to 4 May and renamed みどりの日 or Midori (meaning "green") no Hi (meaning "day"): a celebration of all things green!
In my estimation, this is a most fitting tribute to the late emperor, for apparently he loved nature (I can attest to the fact that though Japan may be one of the most industrialized countries in the world, it is remarkably green, in the conventional sense of the word: forests and mountain ranges remain, as far as I could tell, pristine, perhaps because of the Shinto notion that the spirits lived in the mountains amongst the trees. In terms of the more metaphoric meaning of green, however, I must confess my doubts. The Japanese love gift-giving, and each gift, and indeed many purchases for oneself, are wrapped lovingly in elaborate packaging and paper, all of which goes to waste. How green, as in sustainable, is that practice, no matter how fantastic it may be?).
According to websites, thousands are mobilized on Midori no Hi to plant trees, clean natural sites, and to partake in public-awareness campaigns to educate about environmentalism.
So, today, on our very rainy day here in northern Delaware, go out into your garden, hug a tree (yes, I did just write that), pick up trash you may see littering your way, and rejoice in the greenery--or blues, as the case is in my garden--that surrounds us.