Dickinson never says what the "something" is. Three times the poem begins with the same shrug of a phrase, "A something in a summer's Day," then "noon," then "night," and each time the word stays vague on purpose. The speaker is trying to describe a feeling that language can't hold. The poem follows one day through evening, noon and night, then rolls straight into the next sunrise. Its central idea is that this overwhelming beauty isn't a single event to be grasped and kept. It comes round again, and the poem ends with a welcome for it.
The form carries that feeling of being swept along. The opening stanzas are tercets: two longer lines, then a short one that seems to drop away. "Which solemnizes me." "Transcending ecstasy." "I clap my hands to see." The rhythm is loosely iambic, with the beat of a hymn tune, but the short third lines cut it off like a held breath. Their endings also share a sound. "Me," "ecstasy," "see" and "me" again chime across four stanzas, so the speaker's own "me" keeps coming back, a little overwhelmed each time. Later the rhyme shifts: "bed" in stanza five is answered by "Red" in stanza six. The last stanza stretches to four lines and rhymes "gay" with "Day," which sends us back to the poem's first word.
The feelings change as quickly as the light. At sunset the day's torches ("her flambeaux") burn slowly away, and the response is unexpectedly grave: "solemnizes." Noon is a pile of sensations, "A depth -- an Azure -- a perfume," with dashes that make each noun feel like another attempt to get it right. Night brings something "so transporting bright" that the speaker claps like a child. The imagery doesn't say what the brightness is, though stars would fit. Then comes a shy retreat: "veil my too inspecting face." The next line seems to mean that looking too hard might make the "shimmering grace" flutter off. "Lets" there reads like a compressed "lest," though that is my inference. Either way, the speaker feels that staring at beauty can scare it away.
At stanza five the poem turns inward. The "wizard fingers never rest" may point to nature's magic, but the poem doesn't say whose fingers they are. Then the "purple brook within the breast / Still chafes" its "narrow bed." This is a striking image for feeling that is too big for the body. It suggests blood or pulse as a stream straining against its banks. The next stanza answers it with a landscape: the East raises an "amber Flag" and the sun leads "His Caravan of Red" along the crag. Day is a "she" with torches, the sun a "he" with a caravan, and the poem treats both as living figures. The brook inside and the caravan outside are matching forms of restless, flowing energy. The speaker's feeling isn't separate from the world. It is the same movement happening in a smaller place.
The ending changes the mood. "The night -- the morn / Conclude the wonder gay," and then the speaker meets "Another summer's Day!" It is the only exclamation mark in the poem, and the key word is "Another." The poem began in slow, solemn burning-away. It ends with the dew and a fresh beginning, so what seemed like an ending turns out to be a handover. The wonder is not used up by being felt.
That is what makes the poem worth reading. Words of reverence like "solemnizes" and "Transcending" are attached to an ordinary summer day instead of anything named in a church. The vagueness of "a something" is the most accurate thing the speaker can say. And the dashes, the sudden short lines and the shifts between solemn, childlike and shy all make a reader feel what it is like to be overwhelmed by something and still wanting to look.