A. E. Housman is the poet who wrote "Loveliest of tree, the cherry now."  It is clear from the opening stanza that the poet believes that springtime is the best time for the cherry tree. That is when it is in full bloom with its flowers of white and pink. He thinks that it is so beautiful that it stands out more than any other tree in the woodland that he is riding in. The poet also loves how the tree seems to be celebrating Christ's death and resurrection by "wearing white for Eastertide."  

It is important to note that while cherry trees blossom only for a few short weeks in the spring, the poet still finds the tree beautiful in all seasons. The middle stanza emphasizes the poet's need to take full advantage of the years that he has left on Earth. That concept of carpe diem was very prominent within the genre of romanticism as was the emphasis on the beauty of nature. 

Returning to the poet's love of the tree, he emphasizes its continued white beauty in multiple seasons . . . specifically winter.  "About the woodlands I will go/to see the cherry hung with snow."