Kairouan sits in the interior of Tunisia, away from the coastal breeze, and gets genuinely hot in summer — July and August temperatures can exceed 40°C and make walking the medina uncomfortable. Spring (March to May) and autumn (September to November) are the best times to visit: warm, clear, and manageable. Winter is mild and often very pleasant, though some years bring rain.
The city is busiest during Islamic holidays, particularly Mouloud (the Prophet's birthday), when pilgrims from across Tunisia arrive and the atmosphere in the medina and around the mosques becomes notably more intense. If you are interested in the living religious culture of the city rather than just the monuments, timing a visit around a religious festival can be memorable — though accommodation will be harder to find.
Ramadan is a complicated time: the medina operates on a different schedule, some restaurants are closed during daylight hours, and the atmosphere in the evenings is animated and hospitable. Visiting during Ramadan requires some flexibility but is rewarding for those who are willing to adapt.