When I think of poems about summer, this is the first one that pops into my head. It's one of Shakespeare's most famous sonnets. It compares the person to whom the poem is addressed to a summer day and concludes that although summer may be an uneven and imperfect season ("Sometime too hot the eye of heaven shines..."), for the subject of the poem his/her "eternal summer shall not fade". And it concludes with this famous couplet:
So long lives this, and this gives life to thee."
He was not wrong.
by William Shakespeare
Shall I compare thee to a summer’s day?
Thou art more lovely and more temperate:
Rough winds do shake the darling buds of May,
And summer’s lease hath all too short a date:
Sometime too hot the eye of heaven shines,
And often is his gold complexion dimmed,
And every fair from fair sometime declines,
By chance, or nature’s changing course untrimmed:
But thy eternal summer shall not fade,
Nor lose possession of that fair thou ow’st,
Nor shall death brag thou wander’st in his shade,
When in eternal lines to time thou grow’st,
So long as men can breathe, or eyes can see,
So long lives this, and this gives life to thee.