Past, Present, Future . Events to help the environment or other worthy causes can be at any time locally suitable. As far as weather is concerned, there is no day when weather is more suitable globally than other days. In the United States weather is usually better in April - even better in June when World Environment Day occurs. But on any given day you will find parts of the world that are freezing while others are too hot.
Storms come at different times in different parts of the world.
The advantages of the March Equinox outweigh these difficulties.