___ Day (April celebration)

ARBOR

Think of trees and a springtime holiday that honors planting them.

This clue hints at a holiday celebrated in April called "Arbor Day," which focuses on planting and honoring trees. The blank asks for the first word in the name, so "Arbor" fits perfectly. It’s a straightforward fill-in-the-blank with no wordplay—just a nod to the well-known environmental observance.