One of the leaders of The Family. After Hanunu (the first ruler of Penacony), died of illness, Sunday was invited by the Oak Family to help find a solution to the hypersomnia mutation crisis 

Sunday's proposed solution to the mutation was to use a Stellaron that had been kept under the stewardship of Hanunu. He proposed to activate the Stellaron in order to create a "wedge" in the Dreamscape — a creation of the Enigmata — thereby stabilizing the local area of the Dreamscape. This would enable Penaconians affected by hypersomnia to travel through the Dreamscape and seek refuge, thus allowing them to return to their normal sleep cycle in reality.

The six major families convened to vote on the proposal, which ended in a three-to-three tie. At the instigation of Chachakuka, they tabled the proposal for the time being and moved on to consider other solutions. The next Amber Era coincided with the death of Chachakuka, and the Oak Family became the new rulers of Penacony, who once again turned their attention to the Stellaron... This time, in a ceremony supervised by Sunday himself, Hanunu's orphaned son, Misha, activated the Stellaron and became the Dreamscape Lord, after which the Dreamscape enveloping Penacony gradually became more peaceful. (When he made his wish to the Stellaron, Misha shared some of his authority over the Dreamscape with Watchmaker, which explains the origin of Watchmaker's legacy in the Dreamscape).

The Nightingale Family and the Iris Family responded to the call, and Penacony eventually became the "Planet of Festivities". Mihail realizes that Penacony is heading for a different kind of nightmare, and ultimately decides to seek out power and negotiate with The Family's representative on Penacony — Sunday. Meanwhile, Sunday is eagerly searching for Mihail from the Dreamscape, who is the biggest obstacle to The Family's expansion, and must be eradicated.

On the surface, Sunday is an excellent leader: He seems reasonable, hardworking, an absolute moral model to outsiders, and a natural leader and judge. In the face of (what Sunday considers) virtue, he will offer unconditional acceptance with a big heart. In the face of evil, he will rain down merciless punishment.

On the inside, Sunday is not nearly as kindhearted as he seems. A control freak by nature, he takes pleasure in controlling the behavior and thoughts of others, which is how he became a Family spokesperson. He will coerce, discipline, and even punish others with great effectiveness in order to satisfy his own selfish desires. ( This is not directly shown in the story ) 

Be that as it may, Sunday is not an absolute hypocrite: His great empathy and compassion are real, and he is thus able to guide the public and indeed lead The Family and Penacony upwards for many an Amber Age. He will heartily rejoice in the joys of others and grieve over their sorrows — but that doesn't stop him from fulfilling his selfish desires with a strong hand.