Thursday (full name possibly Thursday Caliban or Thursday Widdershins if he is Captain Widdershins) is mentioned in The End as the husband of Miranda Caliban and the father of Friday. Thursday is a V.F.D. member who is assumed to have sided with the volunteers.
His and Miranda washed up on the Island. Thursday later told Kit how he'd never had better Oysters Rockefeller than during his time on the island; he also informed her of the schism it suffered, implying that he and Miranda arrived when the Baudelaire Parents were still in charge.
Thursday later left the island, presumably due to disliking Ishmael's rule. He left Miranda behind, while she was still pregnant with Friday Caliban. At Ishmael's suggestion, she lied and told her daughter that her father had been devoured by a manatee. (Interestingly, Captain Widdershins told his daughter Fiona that her mother had died in a manatee accident as well, however, it is unknown if these two occurrences are related.)
About six or seven years later, Kit met with Thursday, for turkish coffee, the day before she met the Baudelaire orphans. While he mentioned the schism, she was unaware of how affected the island was when she later arrived.
Miranda implied that Count Olaf was referring to him as "an associate with the name of a weekday" so it is possible that he was in cahoots with him, and not solidly on the fire-fighting side of V.F.D.
- It is unclear if Thursday's last name is Caliban. His wife's surname is Caliban, but it is not revealed whether this is Miranda's married or maiden surname, or if Thursday took her surname.
- If he is a Caliban than he may be related to Olivia Caliban, or he may be related to her through Miranda.
- There is a theory that Mrs. Widdershins and Miranda Caliban are the same, meaning Fiona and Fernald are Friday's siblings or half-siblings (depending if Thursday is Captain Widdershins or someone else.)
- Fernald, Fiona and Friday are all names beginning with F, which could support this theory.
- Another fact supporting this theory is that most siblings in the series come in groups of three, and even on the rare occasions that only two siblings are known of, the existence of a third sibling is usually hinted at (such as the White-Faced Women). The only known siblings in groups of two (aside from Fiona and Fernald) are from All the Wrong Questions: Pip and Squeak Bellerophon, Kellar and Lizzie Haines, Zada and Zora, and Tatiana and Treacle Cozy.
- Something against this theory could be that Kit Snicket believes Friday's father and Miranda's husband, Thursday, is alive and well, and mentions dining with him the day before meeting the Baudelaires. The day before meeting the Baudelaires, Captain Widdershins was with them on his submarine, and by the time Kit was discussing Thursday, Kit believed the Captain to be either deceased or in mortal peril due to the crash of the Queequeg and arrival of the Great Unknown.