SONNET 73

That time of year thou mayst in me behold
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs, where late the sweet birds sang.
In me thou see’st the twilight of such day
As after sunset fadeth in the west;
Which by and by black night doth take away,
Death’s second self, that seals up all in rest.
In me thou see’st the glowing of such fire,
That on the ashes of his youth doth lie,
As the deathbed whereon it must expire,
Consumed with that which it was nourished by.
This thou perceiv’st, which makes thy love more strong,
To love that well which thou must leave ere long.

William Shakespeare

Considering the layout we understand it is a sonnet because it is composed by 14 lines. It is organized into 3 quatrains and one couplet.
In the first quatrain the speaking voice says the beloved that his age is like “time of year,” late autumn, when the leaves have almost completely fallen from the trees and birds stop singing, and the weather has grown cold.
In the following one he compares himself to the "twilight of such day", when the sun goes down to make way to the night. As the sunset his life has almost come to the end, he couldn't sleep anymore because the black night bring away his life.
In the third quatrain, the speaker uses a metaphor comparing himself to the glowing remnants of a fire, which lies “on the ashes of his youth”—that is, on the ashes of the logs that once
enabled it to burn—and which will soon be consumed “by that which it was nourished by”—that is, it will be extinguished as it sinks into the ashes, which its own burning
created.
In the final couplet, the poet tells the young man that he must perceive these things, and that his love must be strengthened by the knowledge that he will soon be parted
from the speaker when the speaker, like the fire, is extinguished by time.
In this poem Shakespeare uses a series of metaphors to characterize the nature of what he perceives to be his old age.
Shakespeare’s anxieties regarding what he perceives to be his advanced age, and develops the theme through a sequence of metaphors each implying something different.
The first quatrain, which employs the metaphor of the winter day, emphasizes the harshness and emptiness of old age, with its boughs shaking against the cold and its “bare ruined choirs” bereft of birdsong. In the second quatrain, the metaphor shifts to that of twilight, and emphasizes not the chill of old age, but rather the gradual fading of the light of youth, as “black night” takes away the light “by and by”.
But in each of these quatrains, with each of these metaphors, the speaker fails to confront the full scope of his problem: both the metaphor of winter and the metaphor of twilight imply cycles, and impose cyclical motions upon the objects of their metaphors, whereas old age is final.
Winter follows spring, but spring will follow winter just as surely; and after the twilight fades, dawn will come again.
In human life, however, the fading of warmth and light is not cyclical; youth will not come again for the speaker.
In the third quatrain, he must resign himself to this fact.
The image of the fire consumed by the ashes of its youth is significant both for its brilliant disposition of the past—the ashes of which eventually snuff out the fire, “consumed by that which it was nourished by”—and for the fact that when the fire is extinguished, it can never be lit again.