April, May, September and October are the best months to visit Tunisia. Spring and autumn bring comfortable temperatures of 20-25 degrees across the coast and interior, the crowds thin out compared to summer, and prices are lower.
Summer (June-August) is peak tourist season for beach resorts like Sousse and Hammamet, but the heat inland and in the south can be brutal -- often above 40 degrees in Tozeur and the desert. If you're mainly beach-bound and don't mind sharing the sand with a lot of Europeans, summer is fine. For exploring Tunis, ancient sites, or the Sahara edge, it's punishing.
Winter (December-February) is mild on the coast but cool and occasionally wet in the north. The south and desert regions become genuinely pleasant -- daytime temperatures of 15-20 degrees make winter the ideal time to explore Douz, Tozeur and Matmata. The International Festival of the Sahara in Douz takes place in December.
Ramadan shifts each year. Many restaurants are closed during the day and the pace of life changes considerably. Some travellers find it an interesting time to experience Tunisian culture; others find the restrictions on eating and drinking in public frustrating. Either way, plan around it rather than being surprised by it.