Generally the idea of “day” is something associated with brightness, sunny skies, and warmth.

The comparison of “youthful day” to “sullied night” is a sharp contrast. These two lines make it clear that our speaker has a great disdain for the fair youth's eventual fall from beauty. Lines eleven and twelve paint this in a stunningly blunt fashion: “Where wasteful Time debateth with Decay / To change your day of youth to sullied night”. The thoughts that our speaker has had in regards to the decay of youth brings into focus the harsh reality that some day, too, the fair youth will lose his beauty. In this final quatrain, our speaker comments on how his reflection upon the shortness of youth has conjured up the image of the fair youth to whom many of Shakespeare's sonnets are dedicated to.

The subject of the sonnet moves from being an impersonal “they” to being a very personal “you” in line ten. Lines nine through twelve become more personal for our speaker. Line eight bears the double meaning of not only being the forgetfulness of elderly people, but also of characterizing those who can't help but relive their glory days every chance they get. Line seven builds to the pinnacle of life, that point when a person is at their absolute prime and they are the best examples of human life before declining into old age. Line seven sets up the beginning of the slide from a youthful prime to an elderly bottom: “Vaunt in their youthful sap, at height decrease” while line eight brings the slide to a close: “And wear their brave state out of memory”. Our speaker revisits this idea later on in the sonnet in lines seven and eight. Here, our speaker recognizes that everything living has a “prime”, there is a brief part of all life that is perfect, where each living thing is at its strongest and most beautiful. The first pair of lines specifically address this issue: “When I consider everything that grows / Holds in perfection but a little moment”. The speaker opens the sonnet with some thoughts on the shortness of life.

Shakespeare sonnet 15