The best time to go to Gzenaya Al Janoubia is during the spring (April to May) and autumn (September to October) months.
During these periods, the weather is pleasantly mild, with warm days and cooler evenings, making it comfortable for exploring the city's historic sites and bustling souks. You'll avoid the intense heat of summer and the chillier, sometimes rainy, days of winter. This allows for a more enjoyable experience when visiting attractions like the Chouara Tannery or wandering through the labyrinthine streets of Fes el-Bali.
These shoulder seasons also offer a good balance of fewer crowds compared to peak summer, while still having all the local businesses and services fully operational.