Cuba has fairly distinct seasons: a relatively dry and mild winter (November–April) and a hot and wet summer (May–October). Early spring is the ideal time to travel, especially in the Oriente (the eastern provinces), which can be insufferably hot in summer. Christmas and New Year’s are the busiest periods, and many accommodations and car rental agencies sell out then, while finding a domestic flight is nearly impossible.
Hotel prices are usually lower in summer—the low season (temporada baja)—when hurricanes are a slim possibility. Tropical storms can lash the island even in winter, however.
You might want to time your visit to coincide with a major festival, such as Carnaval in Santiago de Cuba or the Festival of New Latin American Cinema in Havana [1].
Links:
[1] http://www.moon.com/destinations/cuba/havana/entertainment/events/winter