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 etiologies. The poem is a significant, and in some cases unique, source of fact in studies of religion in ancient Rome. 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.