Santiago de Cuba is a destination which is generally warm during the winter months and can get scorchingly hot during the summer. Between June and October the level of humidity can get quite uncomfortable for some. Very often it has some of Cuba's hottest weather anytime during the year. Within Cuba it is known as "la tierra caliente" The plus side to the area is that the waters surrounding Santiago stay warm enough to enjoy all year long. Wearing long sleeve clothing would only be necessary at night-time, possibly between mid-December to the beginning of March, if at all.
From May to November are the months of the heaviest precipitation. Tropical storms and the possibility of Hurricanes are most likely to occur during the summer until November. http://www.cubahurricanes.org/The all-inclusive beach resorts have their high season December through March. Some of these hotels will close during low season and flights to Santiago will lower in volume which might create a necessity to fly to Havana or Holguin and transfer from there. The city hotels will usually stay open all-year round though.
Santiago hosts quite a few different festivals year-round. You can get more information on the Events and Festivals page.The Fiesta del Fuego Caribbean festival runs at the beginning of July and the Santiago de Cuba Carnival is a very popular Cuban event that takes place near the end of July each year in the city streets. Making accommodation reservations beforehand for hotels or casa particulars (bed & breakfast) is recommended for these events.