of heaven shines, And often is his gold complexion dimm’d": with these two lines, Shakespeare uses the symbol of the sun—"the eye of heaven" and its "gold complexion"—to point out that youth and beauty are not eternal and that they fade, perhaps a bit too quickly.

“And every fair from fair sometime declines, By chance or nature’s changing course untrimm’d": these lines symbolize autumn and change; as leaves turn red and brown, people turn old and gray and their appearance and character are not as vibrant or as lively as before.

The line "Nor shall death brag thou wander’st in his shade" symbolizes death and mortality.

Shakespeare's lover, however, defies nature and transcends natural law; his beauty and youth, more captivating than a summer's day, will never fade and will live on forever, as Shakespeare immortalizes his lover with his words, enabling him to live forever and be eternally young and beautiful in his verse.

My colleagues have answered this very well. I'd like to add a few comments.

Summer traditionally represents the time in life when we are fully blooming. Spring is the virtuous youth. The metaphor he is playing with is the traditional notion that we all live the seasons of man, and that we have the most promise in the spring and are at our hottest (ripest) in the summer. As my colleagues have pointed out, the metaphor breaks down: it is too hot in the summer, while she is temperate. He can't compare her to a rosebud either, because they are vulnerable and apt to be destroyed. These two metaphors refer to physical and spiritual qualities: the summer is the full realization of her beauty (and also, possibly, includes a sexual awakening); the spring speaks of virtue (buds=virgins) as well as promise. Just as the summer is too hot to work as a metaphor, there are problems with the use of the spring/bud metaphor: rough winds. The problem is that in nature, no virtue lasts forever, and every promise of perfection ends in destruction. So the nature metaphor is rejected. He also uses the metaphor of Death's shade to show the possibility that she could be eclipsed by death, that death could hide her or that she would fall into his shadow and be forgotten. The final metaphor compares the eternal spirit (or essence) of a person to the eternal power of the written word and, implicitly, the poet to a god: she lives forever in lines that he writes, after all.

The speaker announces his first simile--a comparison-- in the first line:  Shall I compare thee to a summer's day--and then shows the ways in which his love is even lovelier than this comparison allows. When he says "thy eternal summer shall not fade," he uses a metaphor that suggests she will always be young to him, that she has a glow and vitality that will be everlasting. He personifies Death, claiming "he" will never claim his lover, that she will never die but always live (metaphorically) in his heart. He then says his poetry will give "life" to her, by "life" meaning she will remain immortal on the page, thus comparing physical life to the thoughts created by beautiful words of poetry.

The speaker wonders whether it would be appropriate to compare his love to a summer day, and then proceeds to outline the ways in which she actually exceeds it. First, he says that a summer day is actually not as lovely or temperate as she is. Sometimes "Rough winds [...] shake the darling buds" and "summer's lease hath all too short a date"; in other words, summer can be somewhat extreme, even volatile, and it does not last very long because it eventually gives way to autumn. Further, sometimes summer can be "too hot" and then, other times, the sun goes away for a long time (when "his gold complexion" is "dimm'd"). Furthermore, nature's beauty has a way of "declin[ing]" as time passes. The speaker's love, on the other hand, lives in an "eternal summer" because her beauty will never die; he has immortalized her and her beauty in these lines, and so she is better than literal summer.

The speaker of Shakespeare's Sonnet 18 poses a question in the first line: "Shall I compare thee to a summer's day?" This would seem to be an enormous compliment to the beloved figure here—the "fair youth," to whom this group of sonnets is supposedly dedicated. As the poem goes on to reveal, though, this comparison would not be adequate, as the beloved is far more beautiful even than a summer's day. The beloved is more pleasant than even the nicest weather, as "Rough winds do shake the darling buds of May." That is, May is generally a mild month, but even then it can be windy. When the speaker says in the previous line that the beloved is more "temperate," it is a pun, since weather can be temperate, but the word can also be used to describe someone's mood. The speaker's beloved is not as moody as even a pleasant month like May; they are much more congenial.

In the fourth line, the speaker says that "summer’s lease hath all too short a date." This means that summer, as beautiful as it is, is finite. In the seventh line, he elaborates on this: "every fair from fair sometime declines." It is inevitable for beauty to fade. The beloved's beauty will not fade, though: "But thy eternal summer shall not fade, / Nor lose possession of that fair thou ow’st." Note that the speaker contrasts the finitude of actual summer with the beloved's "eternal summer." Not only will the beloved's beauty not fade, but they will not even die: "Nor shall death brag thou wander’st in his shade, / When in eternal lines to time thou grow’st."

What preserves the beloved's beauty is the very poem the speaker is writing. As long as people can read, the speaker says in the final lines, they will know about the beloved's beauty, and thus, it will live forever. As with many of Shakespeare's sonnets, the piece has a self-reflexive aspect. The poem becomes about the power of poetry.

In "Sonnet 18" the speaker muses whether he should compare the object of his love to the beauty of a summer's day. Yet he soon realizes that such a comparison would not do justice to his paramour. For a summer's day, though undoubtedly beautiful, is, like all aspects of nature, subject to change and decay. When summer comes round each year, we always find that its "lease hath all too short a date;" in other words, it doesn't last for very long.

But the beauty of the speaker's lover—her summer—shall never fade. For her beauty is not of this world; it is a transcendent beauty, a beauty of the soul. And even after she finally departs from this mortal life, her beauty will live on forever, encapsulated in the unforgettable words of "Sonnet 18":

So long as men can breathe or eyes can see, So long lives this, and this gives life to thee.

In the first two quatrains of this poem, Shakespeare compares his love to a summer day in a variety of ways. In each case, summer is less appealing than his love.

He says, for example, that summer is too windy.  He also says that it is sometimes too hot and sometimes too cold.

In the next quatrain, he is talking about his love.  He is saying how she will be different from summer.  She will not fade away quickly like summer does.  She will live forever and death will never claim her.  So I suppose that you would say it refers to his love and her beauty and youth.

What is unusual about the description of summer in Sonnet 18 is that the speaker mentions the negative aspects of summer.  Usually when we think of summer we think of the beautiful sun and warm temperatures, but here the speaker mentions the "rough winds" that shake the early flower buds, and the cloudy skies when the "gold complexion (of the sun is) dimmed."  The speaker comments on the fact that while summer is warm, it can sometimes be too hot to be enjoyable.  The ultimate point he makes about summer is that summer is imperfect and too short of a season, and like everything of nature, it eventually fades to something not as pleasant (like fall and later winter.)  This is what he means when he says that "every fair from fair sometimes declines, By chance of nature's changing course untrimmed."  When you consider the premise he makes in the first line that he is going to compare the person to a summer's day, you might expect a list of pleasant images, but remember that the second line says, "thou art MORE lovely and MORE temperate."  The speaker is paying a very high compliment when one considers the good and the not so good aspects of summer.

In the sonnet, Shakespeare compares his lover to a summer's day, emphasizing the beauty of the summer. The month of May, being a part of summer, he mentions as a beautiful month filled with flowers but with rough winds.

The main point of this sonnet is to illuminate the fact that, as wonderful as the summer is, his lover is far superior. The beauty and warmth of summer is incomprehensibly lesser than his lover, and summer still carries many negative aspects. He mentions the rough winds devastating the flowers and blooms in May, and the fleeting nature of summer days, neither of which apply to his love. He is emphasizing the idea that his lover's beauty never fades, and there is no harshness to detract from his lover's beauty and greatness.

In Shakespeare's sonnet, May is a part of summer; any symbolism in the poem encompasses both of them.

Summer, then, becomes symbolic of nature, which pales in comparison to the one to whom the sonnet is addressed. May is characterized by "rough winds" that destroy new flowers; the summer sun is sometimes too hot and sometimes grows dim; summer itself is too brief and transitory. None of summer's beauty or goodness remains: "And every fair from fair sometime declines." Summer symbolizes the natural order, which includes death. The remainder of the poem explains why the sonnet's honoree possesses an "eternal summer" that "shall not fade" because of the sonnet itself:

So long as men can breathe or eyes can see,

So long lives this, and this gives life to thee.

See the eNotes link below for a discussion of Renaissance writers’ view of the ideal universe and the corruption of nature after Adam’s fall from grace.

The speaker of the poem refers to the "darling buds of May," the flowers that bloom and grow throughout the seasons of spring and summer. He suggests, however, that "Rough winds" also sometimes blow. This can be unpleasant or damaging to the flowers.

Further, the speaker references the fact that summer does not last very long, that its "lease" has "too short a date." Its shortness is another strike against it. Next, the speaker describes the fact that the "eye of heaven," or the sun, sometimes shines too hotly for their comfort. On the other hand, the sun's "gold complexion" is sometimes dimmed by clouds or summer storms.

The speaker outlines many aspects of summer that make it either uncomfortable or less than perfect for some reason. He wishes to show that his beloved is more beautiful and mild even than summer. People often think of summer as the perfect, or at least the best, season. The weather is warm, there is lots of sunshine, and nature seems to be fully alive and in bloom. In spite of all this beauty, the speaker claims that his beloved is more lovely and temperate than a day during this most-perfect season for all of the reasons outlined above.