Spring (March to May) and autumn (September to November) are the most comfortable, with warm days and mild evenings ideal for walking the medina. Summer is hot and dry, with strong midday sun that makes the alleys stifling, while winter is cool and can be wet, though it stays atmospheric and much quieter.
The old medina is a genuine maze where phone signal drops and it is easy to lose your way. Carry an offline map with your riad pinned, keep valuables secure in the crowds, and remember that most 'helpers' who approach you unasked expect payment.
Fez is a conservative, religious city, more so than Morocco's coastal resorts. Modest clothing covering shoulders and knees is respectful, particularly around mosques and madrasas, and non-Muslims should not enter working mosques such as Al-Qarawiyyin.
Summer sun is strong and the enclosed medina lanes trap heat, so carry water, wear a hat and plan the hottest hours for shaded courtyards, a museum or a long lunch. Open viewpoints like the Merenid Tombs are best early or late in the day.