This paper discusses two scanty but complex groups of sources which seem to suggest that Thursday (dies Iovis, that is, Jupiter's Day in the Roman planetary seven-day week) was a day of rest in honour of Jupiter during the later imperial period: a number of ecclesiastical texts from late antique Gaul and Galicia, and three documentary papyri from Oxyrhynchus. The former imply that an unofficial observance of Jupiter's Day, as opposed to the Christian Lord's Day (Sunday), persisted among the populace despite Church opposition to such deviant behaviour. The latter hint at Thursday being a non-working day for official bureaux during the third and early fourth centuries, before the formalization of Sunday as an official day of rest by Constantine in 321. The paper concludes with reflections on the idea that during the later imperial period — as the use of the planetary week became increasingly popular — Thursday became the most important and sacred day in the Roman seven-day week by reason of being the day dedicated to the chief god of the Roman pantheon and, at the same time, the day associated with the astrologically favourable planet that had been named after Jupiter. If Thursday was ever a day of rest recurring on a hebdomadal basis during the later Roman Empire, it was presumably the Judaeo-Christian tradition of the Sabbath and the Lord's Day that provided pagans with the notion of a weekly feast day.