Scholars have long recognized that Ovid’s Fasti was one of the sources for Botticelli’s Primavera. But in most cases, Ovid is treated as one source among many, used primarily to explain what is happening on the right side of the picture. In this article, I argue that Ovid’s Fasti – or more specifically, Books IV and V, the sections that cover the spring months of April and May – represent a key not only to the activity on the right, but to the painting as a whole. The importance of Ovid’s text in turn clarifies the role of Angelo Poliziano in the process, in the creation of the Primavera. In the 1480s, Poliziano devoted a series of lectures to Ovid’s Fasti, which nourished a growing interest in Ovid’s text; Poliziano introduced his course with an extended poem – now lost – in the manner of the Fasti, which may well have been the painter’s immediate source. Ovid’s Fasti was Botticelli’s original point of departure, but Poliziano clearly guided the way.