Cythera, where it's located

In Kythira (also called Cythera or Kythera), an island located south of the Peloponnese and belonging to the group of the Ionian Islands, the climate is Mediterranean, with mild, rainy winters and hot, sunny summers. The daily average temperature ranges from 11 °C (52 °F) in January and February to 25.5 °C (78 °F) in July and August.
In inland areas, there are some hills, at an average altitude of 250/350 meters (800/1,100 feet). The maximum altitude is about 500 meters (1,600 feet). The airport is at 320 meters (1,050 ft) above sea level.
To the southeast of the island, halfway between Kythira and Crete, there is a smaller island, called Antikythira.
Here are the average temperatures.
Kythira - Average temperatures
Min (°C)9910121620222320171411
Max (°C)131314172226292926211815
Min (°F)484850546168727368635752
Max (°F)555557637279848479706459

The rains are not abundant, since they amount to 540 millimeters (21 inches) per year, however, they are concentrated between October and March, and in winter, when they can occur as thunderstorm, they are relatively abundant. In practice, in summer it never rains.
Here is the average precipitation.
Kythira - Average precipitation

Winter, from December to mid-March, is mild and quite rainy, especially in December and January, when precipitation exceeds 100 mm (4 in) per month. In periods of bad weather, the wind can blow strongly. However, there is a moderate amount of sunny days. Despite the southern latitude, cold air masses from the continent can at times arrive here, so there can be cold and windy days, with maximum temperatures below 10 °C (50 °F). Snow is very rare, and occurs only during the most intense cold waves, such as that of March 1987. On colder nights, the temperature usually drops to around freezing (0 °C or 32 °F), although in March 2004 it dropped to -4 °C (25 °F).

Spring, from mid-March to May, is mild; in this season, the rains become gradually more rare, and sunny days become increasingly more frequent.

Summer, from June to mid-September, is hot and sunny, but also breezy. Typically, the air is not very humid because of the prevalence of the northern wind, which comes from the mainland. Usually, the maximum temperature hovers around 28/30 °C (82/86 °F), although there may be hot periods, normally of short duration, with temperatures reaching 34/36 °C (93/97 °F); occasionally, it can reach higher values, around 38 °C (100 °F). In June 2007, which, however, has been exceptional, the temperature reached as high as 41.6 °C (106.9 °F).

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

Cythera, sea and beach

The amount of sunshine in Cythera is great in summer, when clear skies prevail, but it is also good in spring, when days with clear skies are already frequent. On the other hand, from November to February, sunny days alternate with periods of bad weather.
Kythira - Sunshine

The temperature of the sea in Kythira is high enough for swimming from July to September, while in June it is still a bit cool, around 22 °C (72 °F).
Here are the average sea temperatures.
Kythira - Sea temperature
Temp (°C)151515161821232423211817
Temp (°F)595959616470737573706463

Best Time

The best time to go to Kythira for a beach holiday goes from June to mid-September. In the first half of June, it can still be a bit cool in the evening: for safety you can bring a sweatshirt.
If you want to visit the villages, or to go in search of the remains of ancient Greece, or the Byzantine and Venetian monuments, you can choose spring, especially in May, since in summer it can be very hot (though rarely enough). You can also choose autumn, until mid-October, while later the chance of rainy and windy days increases.

Back to the Ionian Islands.