We"re celebrating Arbor Day with some of Yosemite"s most famous residents, the giant sequoia trees of Mariposa Grove. The day champions the planting and preservation of trees and can trace its roots back more than 150 years. In 1872, Nebraska City News editor Julius Sterling Morton proposed a day for locals to plant trees, and the idea grew and grew. Today all 50 states celebrate Arbor Day, most commonly on the last Friday in April, although different states might observe it on the best dates for tree planting in their area.