The short answer
Warm and settled. May highs are about 29 °C and lows about 15 °C, rain is rare, and the evenings are long. It is a good month for the city and the mountains and the rose season in the Dadès valley. In 2027 Eid al-Adha is expected about 16 to 17 May, when many shops and restaurants close for days.
In more detail
Our weather table gives May "warm; roses in the Dadès valley". The rose festival at Kelaât M'Gouna, on the road south towards the gorges, is held in May and fits well into a longer loop to the Sahara. Sunset in mid-May is about 19:20.
By the end of the month middays are hot in the medina, so start early. The Sahara is still workable in May with sunrise and late-afternoon activity, though the heat is building.
Plan around Eid al-Adha. It is the biggest family holiday of the year, expected about 16 to 17 May 2027, though the date depends on the moon. Our research notes call it a poor week for medina-focused trips: most non-hotel restaurants and many shops close for several days, and buses and trains are packed as people travel home. Riads and hotels keep serving guests.