Kefalonia, where it is

The climate of Cephalonia (or Kefalonia), a Greek island of the Ionian Sea, is Mediterranean, with mild, rainy winters and hot, sunny summers. At sea level, the monthly average temperature ranges from 11.5 °C (52.5 °F) in January to 25 °C (77 °F) in August.
Here are the average temperatures in the town of Argostoli.
Cephalonia - Average temperatures
Min (°C)9910111518202119161310
Max (°C)141416182326292927231916
Min (°F)484850525964687066615550
Max (°F)575761647379848481736661

The rains amount to about 700 millimeters (27.5 in) per year, and are concentrated in the period from October to April, while in the summer it rains very rarely.
Here is the average precipitation.
Cephalonia - Average precipitation

In the interior, there are hills and mountains, which culminate in Mount Ainos, 1,628 meters (5,341 feet) high. The mountain slopes are greener, and covered with forests.

Kefalonia, Melissani cave

Winter, from December to mid-March, is mild but rainy, especially in December and January, when 100/110 millimeters (40/43 inches) of rain fall per month. However, there is also a fair amount of sunny days. The island is sheltered from cold spells, so much so that in the plain it never snows, although in the mountains of the interior it can sometimes snow. However, sometimes a cold wind from the north can blow, and in the winter months the minimum temperature in the plains can drop to 0 °C (32 °F).

Spring, from mid-March to May, is mild; the rains become gradually rarer, and sunny days become more and more frequent. May is a good month, before the summer heat.

Summer, from June to mid-September, is hot and sunny: the maximum temperature often exceeds 30 °C (86 °F), and it can even reach 35 °C (95 °F), especially in July and August; at the worst it can reach 40 °C (104 °F). Luckily, sea breezes blow, providing relief, at least when the heat is not excessive, and in any case, you can cool yourself down in the sea. This part of Greece, overlooking the Ionian Sea, is outside the area where the annoying but cooling Meltemi blows; as a consequence, here the sea is warmer (see below).

Autumn, from mid-September to November, is mild (initially even quite warm) and rainy. In October, there can still be many sunny days.

Kefalonia, Myrtos Beach

The amount of sunshine in Kefalonia is great in late spring and summer, when clear skies prevail. In contrast, from November to mid-March, sunny days alternate with periods of bad weather.
Cephalonia - Sunshine

The temperature of the sea in Kefalonia is high enough for swimming from July to September, while in June it is still a bit cool, around 22 °C (72 °F), but all in all acceptable.
Here are the average sea temperatures.
Cephalonia - Sea temperature
Temp (°C)161515161822242625221917
Temp (°F)615959616472757977726663

Best Time

The best time to travel to Kefalonia for a beach holiday runs from June to mid-September.
If you want to go on excursions on the island, the month of May or the first half of October are preferable in order to avoid the heat, at least at low altitude, while the summer is fine for mountain climbing.

Back to the Ionian Islands.