Bossaso is one of the hottest inhabited places on earth in summer. The best time to visit is October to March, when temperatures on the coast range from 25–35°C and the northeast monsoon brings some relief from the heat. The sea is calmer during these months, which matters if you plan to go near the water.
April to September is punishingly hot, with June and July regularly topping 45°C at sea level. The khareef (summer monsoon) brings wind and humidity. The mountains to the south are cooler year-round — if you can base yourself higher up for part of the trip, this helps.
Ramadan affects business hours and restaurant availability significantly; plan accordingly.