Also known as Lady Saturday, or Superior Sorceress of the Upper House, Superior Saturday is the first denizen to have been made by the Architect, thus one of the first beings to exist in the Universe. Saturday is apparently afflicted with Envy, as she seems to envy Sunday's position in the House, believing it to be her right as the first being created by the Architect. She is the ruler of the Upper House, where House Sorcery is studied, and the Keeper of the Sixth Key and the Sixth Part of the Will. A powerful sorceress in her own right, her abilities are supplemented with the power of the Sixth Key, which takes the form of a quill.
Until Superior Saturday, little is known about her appearance. In Drowned Wednesday, when Arthur mistakes Saturday for a male, one of the Raised Rats replies that "Superior Saturday is female, as far as we know".
In Sir Thursday, it is made known that she is Lord Sunday's deputy and that Thursday follows her commands. Throughout Lady Friday, Saturday disrupted travel and communications (though with less success, since the latter is under the authority of the Lower House and the Far Reaches) throughout the House and attempted to assert definitive control over the Middle House by using Lord Sunday's tacit approval and authority. It has recently been revealed that she is responsible for the brainwashing practice known as "washing between the ears", thus subtly explaining its regularity.
Her Noon and Dusk are suspected of committing many heinous deeds, such as the murder of both the former Trustees Mister Monday and Grim Tuesday (however, it was later revealed that Dame Primus was responsible), and Tuesday's Seven Grotesques. Saturday was also behind the creation of the Skinless Boy in Sir Thursday. Her influence is seen through her manipulation of Sir Thursday and her attempts to coax Arthur to give her the Keys to the Kingdom. She is the first member of the Morrow Days to face Arthur and not speak to him, up until their encounter in Lord Sunday.
After losing the Sixth Key to Arthur, she is forced to submit to the Piper and act as his subordinate. She is killed by the Piper with a black blade poisoned with Nothing after Lord Sunday explains that it was Saturday who threw the Piper into Nothing. Though Arthur manages to stop it, it dissolves Saturday's head, killing her.
Saturday is described as a tall, beautiful woman with long, flowing blue hair and a golden circlet on her forehead, similar to an Egyptians.
Though Saturday falls short under Lord Sunday's rule, she is responsible for all the problems the protagonists encounter throughout the series. However, she makes her first true appearance in book 6, which is named after her. Her main goal is to burst a hole into the Incomparable Gardens, Sunday's domain, and overthrow Sunday and become the most powerful Denizen in The House. She has a bitter envy for Lord Sunday, which is her sin - she thinks because she is the oldest Denizen in The House she should be ruler.
- Acid Rain
- The Sixth Key
- Superior Saturday's Dawn never appeared in the series, though mentioned, so it is unknown whether he/she is a villain or not.
- Saturday is the fourth and last main villainess of the series to be introduced, after Dame Primus, Drowned Wednesday and Lady Friday.
- Saturday is the only villain of the series whom TV Tropes considered as a Complete Monster.
Dawn, Noon and Dusk