The best time to visit Saint-Louis is November to April, when the weather is dry and the Harmattan winds from the Sahara have not yet kicked in with full force. Temperatures are warm but manageable — around 25–30°C during the day, cooler at night. The Senegal River is at its lowest and the Langue de Barbarie beaches are accessible and calm.
May to July brings the Saint-Louis Jazz Festival, one of the biggest in Africa, which draws musicians and visitors from across the continent and Europe. The festival has been running since 1993 and fills the island's streets with stages and improvised venues. Book accommodation well in advance if you are coming for this.
The rainy season runs from July to October. Saint-Louis gets less rain than Dakar — it sits at the edge of the Sahel — but July and August can still be hot and humid. The Djoudj National Park is at its best between November and March, when three million migratory birds, including enormous flocks of pelicans and flamingos, are in residence.