Cuba has two seasons. The dry season runs November through April — this is peak time for tourism. Temperatures sit between 22°C and 28°C, humidity is lower, and rain is rare. January and February are the coolest months; March and April warm up pleasantly without being oppressive. December brings the Remedios fireworks festival and a lively atmosphere in the cities.
The wet season runs May through October. It's hot and humid, with daily showers and the risk of hurricanes from July through October. Travel is still possible and the landscape turns lush and green; you'll also encounter far fewer other tourists and prices at casas particulares may be lower. Hurricane hits are not guaranteed — Cuba goes years without a direct strike — but they do happen, and a bad storm can disrupt travel plans significantly.
The very best months are November, December, and February through April. July and August, despite the heat and rain risk, are festive: Santiago de Cuba's Carnival takes over the city in the last week of July, one of the great parties of the Caribbean.