Most visitors tour Greece in the summer to enjoy the Mediterranean Sea, but summers are hot and humid. Consider visiting Greece in the winter, when the temperature is cooler, and fewer tourists make for no or shorter lines at sites, better opportunities for unobstructed photos, and cheaper hotel and restaurant rates. Savings of up to 65% can be had if one travels to Greece in the winter!
If you enjoy swimming, then summer is definitely the best time to visit Greece. However, Delphi is in the mountains where weather is typically cooler than at sea level. Visiting Delphi in the winter is comfortable and affords the luxury of shorter lines and fewer tourists than in summer. Compared to summer, when tourists flock to Greece, having so few tourists makes for great photo opportunities and a no-rush feeling when visiting the sites.
So if you love history, Greek mythology, mountains, and nature, Delphi is a place for you!