Traveling between late April and mid-June or September to mid-October ensures that you'll miss the onslaught of summer tourists. Tourists generally flow into Greece in July and August.
Because April to June and September to October are less busy months, hotel rates decline and places of interest are less crowded, so you'll get a more authentic experience for your money.
The weather in Greece from April to June is temperate and mild, ideal for touring. September and October are still warm, but you'll avoid the unbearably hot summer months of July and August.
Holidays from April to June
If you visit in April, you'll get to experience Easter in Greece, which is a major holiday. May 18th is International Monuments Day, and museums and archaeological sites offer free admission. May also features the famous Flower Festival, while June brings lots of music and sporting festivals.
Holidays from September to October
The Athens Hellenic Festival in mid-September features theater productions and street fairs. September also brings the Athens International Film Festival. Ochi Day, commemorating World War II events, is celebrated throughout all of Greece in October.