Q: thanks!
A: Earth Day is 22 April, and it began in 1970. It is "celebrated" or acknowledged by means of helping solve global environmental problems. For example, cleaning up litter, using public transportation/carpooling, etc. Arbor Day (in the USA) is celebrated on the last Friday of April (in most states, however some states have chosen to observe on different days) and the general celebratory act is to plant a tree.