"A Song for St. Cecilia's Day" begins at the spiritual beginning of the universe. When the universe ("universal frame") was created, order was established. Coinciding with that heavenly order was heavenly harmony. This establishes a connection between the perfection of the spiritual and the perfect harmony. In the first stanza, the universe is brought to life by music. The "diapason" is the sound of a pipe organ or, literally, all the tones; thus, the completion of the first stanza ends with a triumphant chord correlating with the creation of human beings.
In the second stanza, the Biblical version of the origin of music is described, (Genesis 4:21), in which Jubal created music by blowing on a shell. This stanza expresses the power of music to evoke emotion. Music is again compared to a spiritual experience ("celestial") and the speaker notes that it would seem that nothing less than a god must dwell in Jubal's shell. "Less than a god they thought there could not dwell / Within the hollow of that shell" (21-22).
In the third stanza, the speaker describes the sound of trumpets and drums and how they inspire action and possibly motivation for war.
The flute and lute sound like complaints of "hopeless lovers." Violins express jealousy and pain. All sounds and instruments can evoke certain emotions. But the organ, associated with St. Cecilia, creates a heavenly sound.
Notes inspiring holy love,
Notes that wing their heavenly ways
To mend their choirs above. (45-47)
In the seventh stanza, Orpheus is compared with St. Cecilia. Orpheus, according to the myth, could summon the trees from their roots by playing his lyre. But when Cecilia played the organ (and/or sang - her voice being like an organ), an angel appeared at this heavenly sound on earth: "Mistaking earth for heaven."
The last stanza, the Grand Chorus, signals the end of creation (Day of Judgment). The sky is "untuned" at the end of time, meaning that earthly music shall end as "the living die" and only the harmony in heaven will remain: "the dead shall live."
"A Song for St. Cecilia" is about the inspirational, motivational, and spiritual power of music.
One legend supposes that Cecilia created the organ. Another legend says that Cecilia survived multiple executions and was singing the praise of God when she was finally killed.