The speaker compare's someone's hair to a summer's day, and continues to compare her to pleasant weather. The speaker talks about how all things decline, but she does not; she is "untrimm'd" and her "eternal summer shall not fade." The speaker finishes by saying that as long as men exist, she will be beautiful.
The poem exists to praise the speaker's lover. By comparing her to a summer's day he shows his strong emotions for her by linking her to an object, a summer's day, that is often associated with positive emotions. The speaker is speaking directly to his love, though the identity of the speaker and lover is a mystery. The speaker's judgement is clouded by his emotions; because of this, he cannot make an objective judgement on her appearance. The tone of the poem is loving. The speaker conveys his affection for her by complimenting her as "lovely and more temperate."
Throughout the fourteen lines of the sonnet, Shakespeare is constant with his Shakespearean sonnet archetype, which involves an ABABCDCDEFEFGG pattern with the GG being a rhyming couplet. The whole poem is one question followed by a sentence long answer responding to it. Many lines end with semi-colons, indicating a new phrase continuing upon the same subject. the title indicates that it is Shakespeare's eighteenth published sonnet.
The speaker uses an elegant tone, and elegant language as well. The speaker uses an extended metaphor, trying to compare everything beautiful about his love to the most beautiful thing he knows, a summer's day. The speaker personifies a summer's day as a man, with "his gold complexion" and "the eye of heaven shines." By using personification the speaker wishes to build upon the central theme of people loving her.
Not only does the sonnet have a constant rhythm through use of its ABABCDCDEFEFGG rhyme scheme, but it also utilizes iambic pentameter, a common trait of Shakespeare's poetry. In addition, the sonnet uses many soft vowels such as the soft 'a' and the 'ou' sound, in order to draw attention to his love's soft features.