This poem is about the beauties of nature, represented by the cherry tree, and how the poet feels he has really not enough time to appreciate this beauty, because, like all living things, he is fated to die.
The first stanza is a straightforward description of the cherry tree in all its spring-time magnificence, which the poet sees in the wood. It is whimsically described as 'wearing white for Eastertide,'(4) as if it too, like humans, is celebrating a religious festival.
In the second stanza, the mood changes and the focus shifts from the tree to the poet himself, as he falls to musing on the inevitability of death. Casting up the sum of his years, he estimates that he has about fifty years left to live, as he is already twenty; this fits in with the idea, found in the Bible, that the human lifespan equates to about seventy years. So again there are religious overtones here, although now tinged with melancholy at the thought of death, and, significantly, despite the religious references, the poet does not seem to particularly believe in life after death. This undercuts the earlier mention of Easter, which of course celebrates resurrection.
The final stanza expresses the poet's feeling that fifty more years are still much too few to appreciate the beauties of nature. Therefore he decides that he will come to look at the cherry tree not only when it is in full bloom, but also in the dead of winter when it is 'hung with snow'(12). The use of the word 'snow' at the very end of the poem is particularly effective; it refers literally to winter conditions, but also harks back to the first stanza, when the tree was 'hung in bloom'. Thus the natural life cycle is summed up, from springtime and the bloom of life through to death and decay. In spring the tree is clothed in the white of bloom, in winter in the white of snow.
The poem thus celebrates the beauties of nature in all seasons, as well as quietly lamenting the inevitable passing of youth to death for all living things. But the poet does not despair. Rather, he determines to make good use of the time that he does have, and to enjoy the beauty of nature as much as he can.