The primary myth of the poem is that of Endymion, complicated by the countless versions of the story. The basic myth is that the man Endymion, being both beautiful and in a profession that requires him to be in the presence of the moon, is beloved by the goddess Selene. Selene, through her own magic or by requesting help from other gods, puts Endymion to sleep, so that she can love him in dreams. In turn, Endymion falls in love with the goddess while dreaming. In most versions of the story, Endymion never wakes up, and is eternally young in repose. This poem became an inspiration for many visual arts, with the sleeping Endymion visited by the goddess becoming a common theme of painting, sculpture, and carved relief throughout Greek, Roman, and Romantic art.

In his new version of the Endymion myth, with elements pulled from many sources, Keats utilizes both Greek and Roman myths, along with their different spellings (Vesper and Hesperus, Persephone and Porsepine). Romanticism, especially British Romanticism, draws heavily from Eastern Mediterranean mythology, often without distinction, as the classics. By utilizing several different spellings and names for the same beings, Keats evokes the experience of reading different anthologized myths in works such as collected translations of Greek and Latin texts as well as Romantic and Humanist writing such as John Milton’s “Lycidas.