The Fasti, sometimes translated as The Book of Days or On the Roman Calendar, is a six-book Latin poem written by the Roman poet Ovid and published in AD 8. Ovid is believed to have left the Fasti incomplete when he was exiled to Tomis by the emperor Augustus in 8 AD. Written in elegiac couplets and drawing on conventions of Greek and Latin didactic poetry, the Fasti is structured as a series of eye-witness reports and interviews by the first-person vates with Roman deities, who explain the origins of Roman holidays and associated customs—often with multiple aetiologies. The poem is a significant, and in some cases unique, source of fact in studies of religion in ancient Rome; and the influential anthropologist and ritualist J.G. Frazer translated and annotated the work for the Loeb Classical Library series. Each book covers one month, January through June, of the Roman calendar, and was written several years after Julius Caesar replaced the old system of Roman time-keeping with what would come to be known as the Julian calendar.

In ancient Rome, the fasti were chronological or calendar-based lists, or other diachronic records or plans of official and religiously sanctioned events. After Rome's decline, the word fasti continued to be used for similar records in Christian Europe and later Western culture.