Sienna the Saturday Fairy is one installment in the popular Rainbow Magic series, belonging to the sub-series focusing on the Day-of-the-Week Fairies. The story follows human friends Kirsty Tate and Rachel Walker as they embark on a mission to find and rescue Sienna, the fairy responsible for making sure Saturdays are fun, restful, and filled with weekend magic. Sienna has been banished by the wicked Jack Frost, and her absence causes all Saturdays in the human world to become dull, boring, or chaotic with household chores. The girls must use their detective skills to find Sienna and restore her magical charm to bring joy and proper weekend spirit back to the world.