May, June, September, and October are the best months. Daytime temperatures are warm but not punishing (24–28°C), the sea is swimmable, the beaches are not yet jammed, and the bird-watching season at Lake Ichkeul has either just ended or just begun.
July and August are hot, dry, and busy. The corniche fills with Tunisian holidaymakers, the beach hotels are full, the cafés stay open very late. Temperatures sit in the low to mid 30s, with the breeze off the Mediterranean making it tolerable. Book accommodation early in this stretch.
November to March is the rainy and cool season — temperatures drop to 10–15°C in the day, rain is common, and the old town quietens down considerably. This is the time to visit Lake Ichkeul: the wintering flamingos, ducks, and geese arrive from October onwards and stay through March.
The local festival worth knowing about is the Bizerte Carthage Festival of the Mediterranean in July, with music and theatre staged in the harbour and at venues across the city.