Abundance makes people careless, and Dickinson turns that small truth into a miniature comedy of bad timing. Summer, she says, was something "we did not prize," because "her treasures were so easy": free, plentiful, there for the taking. Only now that the season is leaving does it "instruct" us, and the lesson is an old one. We learn what something was worth by watching it go. The phrase "Instructs us by departing" is neat because summer doesn't lecture. It teaches by subtraction.
What lifts the poem above a sigh about autumn is how it treats the slowness of the human mind. "Recognition," the moment of realising what we had, is given a personality. While summer lasted it was "lazy," in no hurry at all. Then, in the second stanza, it "Bestirs itself -- puts on its Coat" and hurries off to "scan" for "Trains that moment out of sight." We can picture a commuter who reaches the platform just as the last carriage slips away. The joke is rueful rather than cruel. Recognition is brisk and well turned out, and completely useless, because it is one minute late.
"Fatal promptness" is the phrase to linger over. Promptness means being on time, yet it is attached to something that is, by definition, too late, and that oxymoron holds the poem's whole argument in two words. "Fatal" adds a note of finality: the promptness comes too late to change anything. The last line, "Unconscious of his smartness," turns the knife. "Smartness" suits the coat and the quick step, but "smart" can also mean a sting, and judging by the tone the second meaning is hard to ignore. The figure is dapper and efficient and has no idea how pointless that is.
A small grammatical shift deepens the picture. Recognition is "it" while it is lazy and abstract ("puts on its Coat"), but by the final line it has become "his." It turns into a person at the very moment it is too late to matter. Summer, meanwhile, is "Her," a generous giver scattering "treasures" without counting them. The poem sets the careless giver against the late-arriving receiver and lets the gap between them carry the feeling.
The form supports this. The poem is a single eight-line sentence, and the stanza break falls between "lazy" and "Bestirs," so the pause comes just before the jolt of action. The dash after "lazy" widens that pause, and the dash after "itself" makes the reader wait a beat before the coat goes on. The first line is a steady, easy iambic tetrameter, as relaxed as the attitude it describes, and "And recognition lazy" shrinks to six syllables so the stanza seems to droop. The rhymes fall on the second and fourth lines: "easy" with "lazy," "promptness" with "smartness." Each pair ends on an unstressed syllable, so the stanzas close softly rather than with a click. Joining "easy" and "lazy" by sound also hints at cause and effect, since the ease of the gift breeds the laziness of the response. "Promptness" and "smartness" then answer them with a brisk, self-satisfied echo. The odd-numbered lines ("prize" and "now," "Coat" and "sight") barely rhyme at all, which leaves the poem pleasantly off-balance.
Dickinson, a nineteenth-century American poet who wrote close to eighteen hundred poems and published only a handful in her lifetime, is known for compressed lines, odd capitals, and dashes that break thought into pieces. All of these appear here in miniature. The poem matters because it takes an abstract noun and gives it a coat, a timetable and a streak of vanity, so that a familiar regret becomes a scene we can see. It also leaves the reader a little uncomfortable. The poem is not really about summer. It is about us, and about how easily we'll be standing on some platform, smartly dressed, watching the smoke thin out over the rails.