Shall I compare thee to a summer’s day? Thou art more lovely and more temperate:
The speaker starts by asking or wondering out loud whether he ought to compare whomever he’s speaking to with a summer’s day.
Instead of musing on that further, he jumps right in, and gives us a thesis of sorts. The object of his description is more "lovely" and more "temperate" than a summer’s day.
"Lovely" is easy enough, but how about that "temperate"? The meaning that comes to mind first is just "even-keeled" or "restrained," but "temperate" also introduces, by way of a double meaning, the theme of internal and external "weather." "Temperate," as you might have heard on the Weather Channel, refers to an area with mild temperatures, but also, in Shakespeare’s time, would have referred to a balance of the "humours."
No need to explain this in great detail, but basically doctors since Ancient Greece had believed that human behavior was dictated by the relative amount of particular kinds of fluids in the body (if you must know, they were blood, yellow bile, black bile, and phlegm. Yummy, no?).
By the early 1600s, this theory was being strongly challenged, but people in Shakespeare’s audience would have known that "temperate" meant that someone had the right amount of those different fluids.
The other important (and less disgusting) issue these lines bring up is the question of "thee." Normally, we’d just assume that the object of the poem is his lover, and leave it at that. But with Shakespeare, these things are always complicated.
What can we tell about the relationship between the speaker and his addressee from the way he addresses "thee"?
For the moment, all we can really tell is this: the speaker doesn’t seem to care much what "thee" thinks. He does ask whether he ought to make this comparison, but he certainly doesn’t wait long (or at all) for an answer.
So is he just wondering out loud here, pretending "thee" is present?
Even better, and this is important, could "thee" also be us readers? Is it just us, or does some small part of you imagine that Shakespeare might be asking you, the reader, whether you want him to compare you to a summer’s day? Keep that on the back burner as you go through the poem.
Finally, just a note on the meter here:
Go ahead and read those first two lines out loud. Notice how they’re kind of bouncy? That’s the iambic pentameter: "compare thee to a summer’s day."
So do you want to see a cool bit of foreshadowing? The pronoun "I" is a stressed syllable in the first line, but the pronoun "Thou" is unstressed in the second line. Guess who’s going to be the real subject of this poem.
Rough winds do shake the darling buds of May, And summer’s lease hath all too short a date:
Here the speaker begins to personify nature. In other words, some of the smack talking he’s doing about summer sounds like he’s talking about a person.
Basically, strong summer winds threaten those new flower buds that popped up in May, and summer just doesn’t last very long.
The way he describes the short summer, though, is what’s interesting. Summer has a "lease" on the weather, just as your family might have a lease on your car; like a person, summer can enter into, and must abide by, agreements.
The point here is clear enough: the summer is fated to end.
But check this out: isn’t summer also fated to begin every year once again? Can the summer possibly have "too short a date," if it happens an infinite number of times? Isn’t it, in a meaningful sense, immortal?
Keep this in mind as you read on.
Sometime too hot the eye of heaven shines, And often is his gold complexion dimm’d;
Here comes the major personification of nature. Put simply, the speaker’s saying sometimes the sun is too hot, and other times you can’t even see it at all (hidden, we assume, by clouds).
But instead of being boring, he calls the sun the "eye of heaven," refers to it using the word "his," and gives it a "complexion," which generally means refers to the skin of the face.
Check out how much more information about the summer we’re getting than we are about the beloved. Indeed, the speaker is carefully describing the summer individually, and even in human terms, while he only describes "thee" in one line and only relative to the summer.
"Complexion," in particular, is especially interesting, as it brings back the whole "humours" theme we saw in "temperate."
"Complexion" used to be used to describe someone’s health, specifically with regard to their balance of humours. Thus, we see here again that the speaker is combining descriptions of external weather phenomena with internal balance.
And every fair from fair sometime declines, By chance, or nature’s changing course untrimm’d;
With these lines, the speaker gets even broader in his philosophy, declaring that everything beautiful must eventually fade away and lose its charm, either by chance or by the natural flow of time. Kind of like teen pop stars.
Now what exactly does "untrimm’d" refer to?
We might read it as what happens to "fair" or beautiful things. By that reading, things that are beautiful eventually lose their trimmings, or their decorations, and thus fade from beauty.
On the other hand, "untrimm’d" is also a term from sailing, as you "trim," or adjust, the sails to take advantage of the wind. This gives "untrimm’d" a completely opposite meaning; instead of "made ugly and plain by natural changes," it means "unchanged in the face of nature’s natural changes."
Here, then, we are subtly prepped for the turn we’re about to see in…