Summers are extremely hot with temperatures well above 40°C in July and August, you should drink plenty of water to keep hydrated which is important in a very hot place such as Fethiye.
Winters are cool with temperatures around 14-20°C. Spring and Autumn are the wettest seasons, yet still very sunny, which is the best time to go for people who want a warm holiday that is not too hot like the summer, with temperatures around 20-28°C.