(Note: I'm not the person who asked this question, but when I submitted my answer Stack Exchange replaced that person with me!)
It is sometimes claimed, even in some fairly recent writings about Halloween, that the ancient precedent for Halloween is a Roman festival dedicated to Pomona, the Roman goddess of fruits and orchards (e.g., Bannatyne, Lesley, Halloween: An American Holiday, an American History (1990), pp. 6-7; Kelley, Ruth, The Book of Hallowe’en: the History of Allhallows Eve (1919), pp. 12-13). Some claim that it was celebrated on November 1, others that it was part of an August 13 (or 23) festival dedicated to Vertumnus (god of the turning seasons) called the Vertumnalia. (Kelley, p. 13, claims the Pomona festival was on November 1, and that the Vertumnalia was on August 23.) In his Metamorphoses, Ovid tells a lovely myth about the romance between Vertumnus and Pomona (14.626-771), but in none of his works does he refer to a festival to her. In fact, there is no evidence for a Pomona festival, and it probably never existed (Morton, Lisa, trick or treat: a history of halloween (2012), pp. 17-18). In modern times, the error seems to go back at least as far as the 18th century English topographer William Hutchinson, who wrote, “The first day of November seems to retain the celebration of a festival to Pomona.” This statement was then uncritically repeated going forward (e.g., in Pakenham, J.A.G., “Pagan Rites and Christian Festivals,” The Canadian Monthly and National Review 10:525-33, at p. 530 (July-December, 1876), and in the December 12, 1903, weekly issue of The Churchman, p. 751). One modern writer suggests that Hutchinson was simply inspired by Ovid’s myth (Morton 2012, pp. 17-18), but this would not account for the alleged date since Ovid mentioned no date or even any festival. More likely the error can be traced to a misreading of a passage in Varro’s De Lingua Latina, which actually specifies August as the month of the Vertumnalia. (Specifically, the passage in question (6.20-21) first describes certain festivals in August, and then in a separate sentence says that the Vertumnalia is observed “at that time” without further specifying a date. The sentence after that goes on to describe a festival in October, the Meditrinalia. The phrase “at that time” refers to August, but it is possible that some readers mistakenly thought it refers to October.) Note: The Wikipedia article on the Vertumnalia (note 4) traces the mistake back to a 1696 book by Laurentius Begerus, but so far I have not been able to get my hands on it, which is why I say that the mistake goes back "at least" as far as Hutchinson.