Time
The central theme of the poem is time. The speaker uses the image of a bright and cheerful summer day to underscore her inability to appreciate moments of beauty as they are happening. As she notes in the middle of the poem, she struggles with this because she constantly finds herself picturing the end of something, even as it is just starting. In this case, she perceives signs of summer's end almost immediately. She describes leaves and thinks of them falling in autumn, and imagines birds flying south for the winter. For the speaker, these moments are difficult to appreciate because she knows that they will pass. She is unhappy because she is already looking ahead to the elapse of this radiant summer scene.
Divinity
One of the poem's other main themes is divinity. Towards the end of the poem, a celestial being talks to the speaker and tries to change her perspective on the passing of the seasons. It critiques her point of view, claiming that she views the world as a place constantly devoid of joyful things. It says she does so because she is always imagining their end. It then informs her that from its omniscient perspective, these moments are all the more beautiful because they are fleeting. It points out that if she could grasp that, even momentarily, she would better appreciate the scene before her. The poem offers the perspective of imagined divinity to show a different view of time. This allows Brontë to push back on the speaker's frustration about the inevitability of summer's end.
Endings
In conversation with the idea of time, endings are another important theme in the poem. The speaker cannot enjoy the beautiful summer scene that is happening in front of her because she knows it will end. She takes note of various small details that already suggest the waning of the season, even as it is just beginning. For the speaker, her premonition of summer's end makes it impossible for her to appreciate the present. While the poem does show a shift in her perspective, it also concludes with a description of the day's end.
