Phoebus

Phoebus, aka Apollo, appears by name in two of Stevens' poems, "New England Verses" and "Notes Toward a Supreme Fiction":
*"Of Phoebus Apothicaire the first beatitude:/ Blessed, who is this nation's multitude" ("New England vii.1-2)
*"And of Phoebus the Tailor the second saying goes:/ Blessed whose beard is cloacked against the snows" ("New England" viii.1-2).
*"Let purple Phoebus lie in umber harvest" ("Notes" i.14).
*Let Phoebus slumber and die in autumn umber" ("Notes" i.15).
*"Phoebus is dead, ephebe, by Phoebus was/ a name for something that never could be named" ("Notes" i.16-17).
Phoebus is also alluded to in the opening paragraphs of Stevens' essay "The Noble Rider and the Sound of Words," where Stevens quotes Plato's Phaedrus: "'let our figure be of a composite nature--a pair of winged horses and a charioteer'" (Wallace Stevens: Collected Poetry and Prose 643). The image here recalls Helios, charioteer of the sun, frequently synonymous with Phoebus/Apollo in classical mythology. 
==Who is Phoebus?==
As mentioned above, Phoebus is the Greek name for the clasical god also known, in Latin, as Apollo. He is the god of the sun, associated also with medicine, prophecy, art and music, and more. It's important to note that, in the ancient world, Phoebus became increasingly syncretic with Helios, the Titan and charioteer of the sun, who with his steeds Pyrois, Aeos, Aethon, and Phlegon, drove the sun westward across the sky each day and, passing beneath the earth, returned it to the East at night. 
==What is Pheobus doing in Stevens?==
Phoebus, as Stevens himself puts it, is simply "a name for something that never could be named" ("Notes Toward A Supreme Fiction" i.17). The line recalls the famous dictum, from the Tao Te Ching: "The Toa that can be named is not hte true Tao, the name that can be names is not the eternal name." Here, the Tao, or Way, refers to the way of being in harmony with nature, a perfected mode of existence. A similar notion surfaces in Judeo-Christian mythology, wherein the name of God, the ineffable source of being, is unspeakable. 
For Stevens, then, the name "Phoebus", and presumably the word "God", function only as placeholders, signposts gesturing toward the transcendental signified: absolute reality, or the ultimate ground of existence. In Kantian terms, we are speaking of the noumenal world, that place (though place is not really an apposite word here) where one might encounter things as they are.  In Stevens, the noumenal is the opposite of the perceptual or phenomenal, and thus provides both a foundation and a foil for the poetic imagination. Just as Stevens frequently uses the moon, or night, or twilight, to symbolize the poetic imagination, so he also uses the sun, day, or noon, to symbolize ultimate reality. With this in mind, it is hardly surprising that he would also link the real (the noumenal) with Phoebus/Apollo/Helios, the God of light and truth. Further, because Phoebus is also the god of music and poetry, he provides Stevens' with a convenient double-metaphor, one that can symbolize not only the noumenal and phenomenal, but can gesture towards a kind of sublime alchemical fusion between them. 
This last point (above) may help to explain Stevens' attraction to the passage from Plato's Phaedrus, in which Plato alludes to the story of Phaethon, the son of Apollo who demanded he be allowed to drive Apollo's chariot for a day and subsequently lost control of the horses, scorching the Earth. For Plato, the metaphor is an apt illustration of humanities' potential for both nobility and hubris; while for Stevens' it becomes an apt metaphor for poetry's potential to articulate the sublime, or, on the other hand, to fall off into "'gorgeous nonsense'" (Coleridge qtd. in Wallace Stevens: Collected Poetry and Prose 643).