What is "National Arbor Day?" It is a holiday on which trees are honored. The origin of Arbor Day lies in the 19th century. The driving force behind National Arbor Day was J. Sterling Morton. The history of Arbor Day is a history of the celebration of the importance of trees to human life. The history of Arbor Day is thus part of the modern movement towards awareness that we may need to cultivate "nature" a bit in order to preserve it.
The "arbor" in "Arbor Day" comes from the Latin word for "tree," arbor. But the word signifying the trellising structures known as "garden arbors" has a different origin. It derives from the Old French, erbier, which means "garden." But the coincidental similarity in spelling between the Latin and French terms led writers of English to exploit the connection and use "arbor" as the spelling for the trellising structures that grace our gardens.
The official Arbor Day Web site relates the full history of Arbor Day celebrations. According to this site, it was in Nebraska, on January 4, 1872, that J. Sterling Morton first proposed a holiday for tree-planting. Nebraska, mind you, was a more or less tree-less state at the time. Morton's proposal was adopted, and the idea of observing such a holiday spread to other states later in the 1870s. States today most commonly observe the National Arbor Day holiday on the last Friday in April. The observance of National Arbor Day has even spread to countries outside of the U.S.
Unlike the storied origins of many holidays, the history of Arbor Day is the tale of a modern celebration. Nonetheless, while the history of Arbor Day may be short, the pre-history of Arbor Day, if we may call it that, recedes into the mists of the distant past. The reverence for trees (e.g., holly trees) among the ancient Celts may come to mind first, but it was the Norse who held that an ash tree supported the very universe! Arbor Day harkens back to a time when people lived in closer communion with nature -- and gave trees the respect due to them.