Kefalonia, where it is

The climate of Cephalonia (or Kefalonia), a Greek island of the Ionian Sea, is Mediterranean, with mild and rainy winters, and hot and sunny summers. In the plains, the average temperature of January is 11.5 °C (52.5 °F), that of August is 25 °C (77 °F).
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 millimetres (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 metres (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 millimetres (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 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, therefore, 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 basically acceptable.
Here are the average sea temperatures.
Cephalonia - Sea temperature
Temp (°C)161515161822242625221917
Temp (°F)615959616472757977726663

When to go

The best time to travel to Kefalonia, for swimming and sunbathing, runs from June to mid-September.
For excursions and walks on the island, the month of May or the first half of October are preferable, at least at low altitude, to avoid the heat, while the summer is fine to climb mountains.

Back to the Ionian Islands.