Santiago has a tropical climate and is hot year-round. The coolest and driest months are November through April, when temperatures hover around 25–28°C — the most comfortable time to visit. The wet season runs May through October, with heavy afternoon rains that clear quickly.
The standout reason to visit at a specific time is Carnaval, held each July — the biggest and most exuberant carnival in Cuba, a week of conga lines, rum, and dancing in the streets that overwhelms the city and makes Havana's celebrations look restrained. Book accommodation months in advance if you plan to be there for Carnaval. The city is at its liveliest and most itself.
Hurricane season peaks August through October. Santiago has been hit hard in recent decades — Hurricanes Sandy (2012) and Matthew (2016) both caused serious damage. Travel insurance and flexible bookings are sensible during these months.