Arbor Day

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. 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.

Century Dictionary and Cyclopedia

1. n. In some of the United States, a day of each year set apart by law for the general planting of trees wherever they are needed.

Wiktionary

1. n. US The national holiday celebrated by planting a tree; the last Friday in April.

GNU Webster's 1913

1. adj. [U.S.] a day appointed for planting trees and shrubs.

WordNet 3.0

1. n. a day designated for planting trees