What line of the poem shall I compare thee to a summer's day is comparing the warmth of the person to the summer day?

Sonnet 18 by Shakespeare is also frequently called "Shall I compare thee to a summer day". This sonnet does not have similes in it--it is a metaphor. A simile is when you compare one thing to another using "as" or "like" (The room was as hot as the sun). In sonnet 18 the narrator describes a summer day but is actually talking about all the ways in which the object of the sonnet is pleasing to him.