The first line of “Sonnet 73” introduces many of its central thematic and formal qualities. Essentially, the speaker is talking to some unknown addressee and saying, "When you look at me you see a certain time of year." What specific time of year (a.k.a. season) isn't revealed until the next line, but it's clear that this poem will be dealing with the nature and passage of time in some way. Already, the poem is highlighting its metaphorical landscape: this is not a poem about nature, but one that uses nature to speak about human beings.
This first line also hints at the human relationship that grounds the poem. Through the phrase “in me,” which will be repeated as an anaphora in each quatrain, the speaker establishes the poem’s first-person perspective. But the use of “thou” shows that the poem is framed as an apostrophe to a specific listener. Despite how old-fashioned it might sound, "thou" is actually the informal form of the second person singular (that is, of "you"), which suggests that the speaker and the addressee have an intimate relationship.
Finally, the line establishes the poem's use of iambic pentameter, the classic da DUM rhythm one would expect in a Shakespearean sonnet. The first line follows the even rise and fall of its chosen metrical pattern flawlessly (which, in hindsight, may reflect the aging speaker's skill having been accumulated over time). The line is divided into five feet (the basic unit of poetic measurement), each one consisting of an unstressed syllable followed by a stressed syllable:
That time | of year | thou mayst | in me | behold
For the most part, the rest of the poem will follow this steady rhythm. The line is also enjambed, a formal quality that helps create a coherent extended metaphor within each quatrain as the lines overflow from one to the next.