What does "wearing white" symbolize in A.E. Housman's "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.

Why is the tree in "Loveliest of Trees, the Cherry Now" described as wearing white?

Cherry trees have white flowers when they bloom in the springtime. This is why places like Washington, DC celebrate Cherry Blossom Festivals in the springtime. If allowed to mature, the flowers are fertilized and become cherry fruits in the fall.

The poet is talking about trees being observed at "Eastertide," which means it is in the season of spring. The trees are wearing their white cherry blossoms as part of their yearly life cycle, as a symbol of purity at Easter, and as a mark of the changing seasons - they're wearing white cherry blossoms in the spring, as contrasted with the white snow of winter that they also may wear. This could also be considered symbolic of approaching death, since the poet is thinking about the declining number of years remaining in his life.