As everyone is more and more aware, the weather patterns are slightly changing around the globe. Here are a few items of interest to give you a little education of Greece, if that is your vacation destination.
Greece can be divided into sections easily. The northern section consisting of Northern Macedonia (where Alexander the Great was born) and Northern Epiros are like the Balkans with freezing winters and hot humid summers. Going south you have the more Mediterranean climate of mild winters and hot dry summers associated with vacationing in Greece. If this is more to your taste, arrange vacations in the Cyclades, (think Santorini) and Dodecanese islands (think Rhodes), the eastern central Peloponnese (think Epidurus) or the largest most southern island of Crete.
Vacationers that come only in the summer months usually do not realize that there are seasons in Greece, with snow in the winter (Crete's has snow skiing on it's central mountain. Even Athens can have snow and 'that's' a beautiful site when you can see the Partheon in white. Snow is rare in the Cyclades.
In the 'key summer season', July and August, be ready for temperatures that can soar to 40 degrees C (over 100 degrees F). That's why it is always advisable to wear loose fitting thin material clothing, a hat, and have sunscreen and water with you at all times. During these two month there is a strong northerly wind, the meltemi, that sweeps the eastern coast of mainland Greece and the Aegean/Cyclades Islands (Naxos, Amorgos). Just be aware of this as it can disrupt the ferry schedule... Learn the Greek word 'Tha Dhuma" (we'll see) and you will have the right attitude.
If you want to not bother with the dry hot winds of summer, then the Ionian Islands would be good for your vacation. They have less severe winters than northern Greece but are the areas of the highest rainfall. Crete stays the warmest the longest and you can even swim off the island's southern coast (Lybian Sea) from April until November.
There is a rhythm to life here in Greece. If you are to vacation in Sept you might witness the figs and grapes being gathered for wine making, Oct (brings the rain) and the walnuts and chestnuts are harvested. The weather will stay cold and wet from Oct until February. Although there are many day with crystal clear blue skies as well (but you will be wearing coats.
When to go
Most vacationers and Mediterranean cruisers come to Greece in the 3 seasons excluding winter. The KEY months are May, June, July, (August is really hot and crowded with Greek tourists) September and October. **NOTE: If you are planning on doing independent traveling around Greece, know that in the winter months most of the country and the island pretty much close until re-opening in April. Ferry boat schedule and airplanes to islands are much fewer that in the summer months.
Around Pasca (the Orthodox Easter) the first tourists start to arrive so, things are starting the process of opening back up. Weather is beautifully warm and cool at night. Beaches and ancient sites are uncrowded so if you like this come from Easter to mid-June. Figure just the opposite in July and August. **Note: make your reservations ahead of time for tours/guides/and hotels.