William Shakespeare's "Sonnet 73," which begins, "That time of year thou may'st in me behold," addresses the theme of love in light of human mortality. 

The poem is constructed as a typical English sonnet, consisting of three quatrains followed by a couplet, with a major structural turn or reversal between the third quatrain and the couplet. The poem is written in the first person and addressed to a beloved referred to as "you"; the only real detail we learn of the beloved is that the beloved is younger than the narrator.

The three quatrains are an extended meditation on aging. The first quatrain compares human aging to the season of autumn. The second quatrain compares aging and death to the fading of daylight into night. The third quatrain compares aging to a fire burning down from flames to embers. 

The twist in the couplet is that the narrator argues that the fleeting nature of human life strengthens rather than weakens the addressee's love for the narrator:

   This thou perceivest, which makes thy love more strong,
   To love that well which thou must leave ere long.

The theme of William Shakespeare's Sonnet 73 is the importance of the friend of the poet's loving him more strongly because of the temporal state of life.  Calling attention to his aging in order to convince his lover of the urgency of full affection, the poet uses images such as "yellow leaves" and "twilight."  And, with the prefix twi-- which means "half," the poet suggests that his life is nearing its completion.  As the sonnet develops, the suggestion of death comes in the second quatrain:

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.

Further, the ending couplet summarizes the purpose of this sonnet; that is, the poet pleas with his lover to love him more strongly since there is so little time left to him.

In summary, the theme of the importance of the lover devoting attention to the poet is expressed in three metaphors:

1. the yellowing leaves
2. the day fading in the west after sunset and black night coming
3. the glowing of the fire of life being consumed by ashes

What message is Shakespeare trying to convey in Sonnet 73?

Like many of his other sonnets, Sonnet 73 focuses on Shakespeare's anxieties concerning old age.  In the poem, the speaker evokes seasonal imagery to reflect the passage of time by using several different metaphors to compare his advanced age with that of winter.  His tone is grim but resigned as he compares himself to a leafless tree; instead of sweet bird song, there is only "bare ruined choirs" (4). 

Shakespeare uses a gripping metaphor in the third stanza as the speaker compares his life to that of a "glowing fire;" the connotation of his choice of adjective, "glowing" suggests that the fire is steadily dwindling.  He uses the phrase "ashes of youth," that comes with an enormous sense of finality (10).  His life's fire has been reduced to ashes and cannot be rekindled.  As with many of Shakespeare's sonnet, the final couplet reveals the speaker's primary purpose.  He encourages his loved one to appreciate their time together because he must "leave ere long" (12).