The best time to visit Fethiye is April to June or September to October. The weather is warm and sunny, the sea is swimmable, and the crowds are manageable. April and May are especially good — wildflowers are out, the Lycian sites are uncrowded, and prices are lower.
July and August are hot — regularly above 35°C — and crowded with European package tourists. The beaches are packed and prices peak. It is still a perfectly viable time to visit if you like heat, but the shoulder seasons are more pleasant.
Winters are mild (10-15°C) and rainy. The town is quiet and many tourist-oriented businesses close, but the restaurants and fish market stay open, and Fethiye retains a local, unhurried atmosphere that some visitors prefer.