Back in 2003, a book authored by Joshua Muravchik entitled Heaven on Earth: The Rise and Fall of Socialism was published.
In his book, Muravchik demonstrates how Socialism spread faster than any religion and ended up collapsing of its own weight. Environmentalism is grounded in socialism since, like religion, both socialism and environmentalism are based on the morality of self-sacrifice (i.e. altruism).
May Day is known as International Workers Day. It is a day Leftists use to celebrate their (ahem) accomplishments. It is important, instead, to remember and mourn the victims of the evil ideology of Marxism which is the root or basis of environmentalism.