May, June, September, and October are the best months. The weather is reliably warm and sunny but not crushingly hot, the sea is warm enough to swim, and Ephesus is bearable in the middle of the day.
July and August are very hot — afternoon temperatures regularly above 35°C — and the town is at its busiest. Several cruise ships may dock on the same day, which means several thousand extra people in the old town between mid-morning and late afternoon. Combine this with Turkish school holidays and the place can feel overwhelming. If you must come in high summer, plan your archaeological visits for the first hour after the sites open.
Winter (November to March) is mild but quiet — many restaurants and hotels in the resort area close. Some classical sites stay open year-round but with shorter hours and fewer services.