Fairies hide inside a disco ball during the day, and when darkness falls they burst out to dance across the city. That's the whole story, and it's told with a light touch. The disco ball hangs in a "portal to the kingdom of odd," a phrase that turns an ordinary party prop into a doorway between worlds. Once the fairies are out, the city becomes their dance floor.
The speaker doesn't argue for any of this or explain it. They simply watch, and the tone suggests delight. Words like "lickety-split" and "splendid" feel playful and a little giddy. The final line adds a nice twist: the fairies' magenta wings catch the light of "streets, and traffic, and stars," so car headlights and stars shine on equal terms. The poem's mood is whimsical and joyful, as if the plain, busy city at night were secretly full of magic for anyone willing to notice it.