Time
The poem's central theme concerns the passage of time. Dickinson makes a series of elaborate comparisons to cycles in nature to explore this idea. In showing the reader scenes from various times of the day, she is revealing the quiet way that time moves both "imperceptibly" and inevitably. Relying on images of different seasons and times of day, Dickinson is able to elucidate the way in which these things change without the notice of individuals. In the same manner as these smaller occurrences, time is portrayed in the poem as constantly moving, but never in a way that catches our attention. This is most apparent in the poem's final line about summer's "great escape." The speaker marvels at the beauty and grace of a summer day, but feels it slowly slipping away from her. As time passes, the summer is gone.
Loss
The opening line ("As imperceptibly as Grief") sets up a reading of the poem as a meditation on loss and mourning. While much of the text is concerned with natural imagery and abstract ideas around the passage of time, much of its core is about grieving. Dickinson mentions the concept of grief in the first line because, in this poem, ideas about time are inherently bound up in loss. Much like the seasons and daylight, time causes things to fade and vanish. So even as the poem shows the subtlety with which time is passing, it also deals with how people reckon with the loss caused by time's passage. If time is always moving, it is simultaneously creating change. As shown in the poem's final lines about summer's departure, this passage is always bittersweet, constantly creating exits for beloved figures.
The Permanence of Nature
One of the poem's major themes is the fixity of natural cycles. This is threaded throughout the text in the images of different times of day. However, it really comes to the fore of the piece when the speaker imagines summer as a personified guest at a party. Even though the speaker would clearly like "her" to stay, summer must depart, as is dictated by the timing of natural cycles. Her departure is predetermined and unavoidable. It is the way things have always been and will continue to be. This scene encapsulates the idea that nature has a fixed inevitability built into its many rhythms.
