Mykonos in August is a different island from Mykonos in June. Choose the version you want: swimming and lanes, or peak nightlife and peak rates.
June and September
Swimmable water, open hotels, and town that still feels like a village in the morning. The strongest window for most first-timers who are not coming for clubs. September often has the warmest sea.
July–August
The famous season: beach clubs, prices, tender crowds, and nightlife that runs late. Book beds and popular restaurants far ahead. This is a valid Mykonos — just not a quiet one.
May and October
Shoulder edges. Some beach businesses are opening or closing. Fine if you want Chora and a walk to the windmills more than a sunbed. Water can still be cool in May.
Winter
Many venues close. Only for a very quiet town walk. Combine with Athens rather than treating Mykonos as a winter island holiday.