A stamp commemorating Arbor Day.
Arbor Day definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a tree-planting day observed individually by the states of the U.S., usually in spring
Arbor Day definition by American Heritage Dictionary
An unofficial holiday observed in all 50 states of the United States, most often on the last Friday in April, for the public planting of trees.