So"The People's Cube" humorously observes. Closet communist Pete Seeger was a keynote speaker and performer at the original event held in Washington DC, and that is enough to give me pause.
But seriously, why was April 22 chosen? Why not the Spring Equinox, which is when the United Nations, hardly an organization known for opposing communism, celebrates it?
Or around Easter Week if we wanted to make Earth Day a reminder to be God's good stewards of the Earth? Nope, we can't have that, eco-hysteria is a jealous God and will have no gods before it. When people stop believing in God, as Chesterton observed, the danger is not that they will believe in nothing else, but that they will believe in anything else--like a New Ice Age that reverses itself to become "man made global warming" in less than 30 years.
Wikipedia observes that Earth Day may have taken over Arbor Day, which is a plausible explanation:
"April 22 is also the birthday of Julius Sterling Morton, the founder of Arbor Day, a national tree-planting holiday started in 1872. Arbor Day became a legal holiday in Nebraska in 1885, to be permanently observed on April 22. According to the National Arbor Day Foundation "the most common day for the state observances is the last Friday in April . . . but a number of state Arbor Days are at other times to coincide with the best tree planting weather." It has since been largely eclipsed by the more widely observed Earth Day, except in Nebraska, where it originated."
(from Arbor Day's Beginnings. The National Arbor Day Foundation. Retrieved on 2007-04-22).