From Wiki Heraklion
The climate in Crete is temperate. The atmosphere can be quite humid, depending on the proximity to the sea. The winter is fairly mild and tolerable. Snow fall is practically unknown to the plains, but quite frequent in the mountains. During summer, average temperatures are in the high 20's-low 30's (Celsius). Probably the best time to visit Crete is spring and autumn.
Crete straddles two climatic zones, the Mediterranean and the North African, mainly falling within the former. As such, the climate in Crete is primarily temperate. The atmosphere can be quite humid, depending on the proximity to the sea, while winter is fairly mild. Snowfall is common on the mountains between November and May, but rare in the low lying areas, especially near the coast when it only stays on the ground for a few minutes or hours. However, a truly exceptional cold snap swept the island in February 2004, during which period the whole island was blanketed with snow. During the Cretan summer, average temperatures reach the high 20s-low 30s Celsius (mid 80s to mid 90s Fahrenheit), with maxima touching the upper 30s to mid 40s (above 110 Fahrenheit).
The south coast, including the Mesara Plain and Asterousia Mountains, falls in the North African climatic zone, and thus enjoys significantly more sunny days and high temperatures throughout the year. In southern Crete date palms bear fruit and swallows remain year around, not migrating to Africa.