as an annual ecological awareness event on April 22, dates to 1970; the idea and the name formed in 1969.
Examples from the Web for earth day
No water in fountains, water in toilets only during part of an Earth-day.The Planet Strappers|Raymond Zinke Gallun
A dark shadow blotted out the brightness of the Earth-day for him.Slaves of Mercury|Nat Schachner
Despite the heavy cloud cover, the afternoon was as bright as an Earth-day.Wind|Charles Louis Fontenay
The local day would be something more than an Earth-day in length.Operation: Outer Space|William Fitzgerald Jenkins