"Piedra de sol" or "Sunstone" refers to the round, gigantic, stone Aztec calendars. The poem is inspired by the circularity of the Aztec year, and the way that events repeat in a cycle. Therefore, the poem is circular—it begins and ends the same way, and also goes through a cycle of death and rebirth, like the seasons of the calendar. In the Venus-based calendar (there were a few different calendars, but this one measured Venus' orbit), there were 584 days in a year—the exact number of lines in this poem (in the original Spanish). Nifty, no?
There are a lot of ways to think about why Paz might have decided to structure his poem this way. Since the calendar is based on Venus, and the poem is about a forgotten/remembered love, there might be an erotic connection between the love goddess Venus and the beloved in the poem.
Another explanation is that the poem itself symbolizes death and rebirth in a never-ending cycle. The poem's getting back to the forgotten moment relies on the belief that death isn't the end—that there is a way to be reunited with the beloved. The calendar, with its repetition every year, is a way of finding that reassurance.
And one more thing we could think about is the way that Aztec culture is, in some ways, buried in Mexico's history, like the not-quite-forgotten affair in the poem. The Spaniards may have conquered America, but they didn't wipe its history off the map, and the poem touches those cultural memories as a way of connecting with the past.
These are just a few interpretations of the title and structure—can you think of any other good reasons that the poem might be based on the calendar?