The best time to visit, weather-wise, is November through March. Days are warm (23–32°C / 73–90°F) and the humidity is lower. Nights in the mountains and highlands can be noticeably cool.
The rainy season runs from April to June and again from October to November. Rain is typically short and heavy — an afternoon downpour rather than all-day drizzle — but roads can become impassable, particularly in rural areas.
Hurricane season is June through November, with the peak in August and September. Haiti is particularly vulnerable: deforestation has stripped hillsides across much of the country, making flooding and landslides far more severe than in neighboring islands. Hurricane Jeanne (2004), Hurricane Hanna (2008), and Hurricane Matthew (2016) each caused devastating casualties and damage.
Carnival, held in February or early March before Lent, is the high point of the social calendar. Port-au-Prince and Jacmel both hold major celebrations. The Guédé festival in early November — honouring the Vodou spirits of the dead — draws large crowds to the National Cemetery in Port-au-Prince and is one of the most striking cultural events in the Caribbean.
If you are traveling specifically for cultural events, February (carnival) or early November (Guédé) are the most rewarding times to visit — assuming conditions allow.