The poem treats ordinary living as a kind of religious ritual. The first line announces a holy day, but the only thing "observed" is being alive. There is no church, no ceremony, just the day itself, and the speaker seems quietly glad about that.
The second stanza turns earthy and a little funny. A gland, an organ most of us never think about, becomes a meal, "so full and so giving." The body is something to be eaten and shared, and the language makes that feel generous rather than gross. It suggests a world where everything, even the humble or unappealing, offers nourishment.
The last two lines are the strangest. Asking who we should "age into / if not a shoe" imagines growing old as becoming something worn, comfortable, and useful, shaped by walking through the world. The mood is playful, humble, and warm, with a calm acceptance of aging and the body.