What is the symbolism of "wearing white" in A.E. Housman's poem "Loveliest of Trees, the Cherry Now"?

This is not exactly symbolism but personification. The poet is suggesting that the cherry tree covered with white blossoms is like a young woman wearing a white dress for Easter. A white dress traditionally might also suggest marriage and rebirth, too—just as the cherry tree's blossoms are a herald of spring, mating, and new life.
The poem seems to be a compliment to the beauty of the cherry tree and the speaker's reflections on the joy of a season traditionally associated with youth. The girl who wears the white dress at the first sign of spring experiences spring as a young person, while the speaker of the poem contemplates the brevity of life and the beauty of nature.