Mykonos is blessed with a beautiful weather for almost 300 days of the year. It rarely rains in summer and you can enjoy the swim and the beach for all year long. Showers are anticipated from October-end to April.
June-August: If you are fond of summers and sunbaths, this is a great season where you pick a lot of fests and parties. Being the peak season though, the deals are highly priced and making early reservations would be the best advice.
September-October: This being the post-season period, the rates are fair and the waters warm enough to have a nice swim. The place will be less crowded and this is probably the best time to be in Mykonos.
April-May: This is the pre-season period when the rains are less prevalent and the rates for hotels and travelling are fairly low.
Vacation in Mykonos is a one big party. And duh, do we need to look for a perfect time for the same!