Map from Google - Athens

In Athens, capital of Greece, the climate is Mediterranean, with mild and moderately rainy winters, and hot, sunny summers. The city is located near the 38th parallel, but in winter, from December to mid-March, it's sometimes subject to outbreaks of cold air from the Balkan Peninsula, which can bring cold and rain for some days, and sometimes even snow. Snowfalls occur more easily in the north of the city; at the centre, the Acropolis is whitened by snow more easily than then lower surrounding areas; a particularly abundant snowfall, which affected also the southern and coastal areas, occurred in February 2008.
Summer is hot and sunny; in summer Athens is the hottest capital of Europe; in July and August the temperature reaches quite easily 35/36 °C, but sometimes even 40 °C.
Precipitation is not abundant, since it amounts to about 400 millimetres per year, and follows a Mediterranean pattern, with a maximum from November to February, and a minimum in summer, when clear skies prevail and there are occasional thunderstorms.
Here is the average precipitation in the Ellinikon Airport.
Average precipitation Ellinikon
Ellinikon J F M A M J J A S O N D Year
Prec. (mm) 50 40 40 25 15 6 5 7 10 50 55 65 368
Days 9 7 8 6 4 1 1 1 3 5 8 11 64

The city covers a wide area in the plain of Attica, and is surrounded by mountains on three sides, and open to the sea to the south. Thus, there are slight differences that are worth considering, looking at data of four stations, from north to south.

Athens, weather stations position

Let's start from the Tatoi Airport, located in Decelea, at 240 metres above sea level, on the northern outskirts of the city, close to the foothills of the Parnitha mountain range. Here the average temperature of January is 7.5 °C, that of July is 25.7 °C; precipitation amounts to 450 mm per year, with a maximum of 85 mm in December and a minimum of 6 mm in August. Here are the average temperatures.
Tatoi Airport average temperatures
Tatoi Airport J F M A M J J A S O N D
Min (°C) 3 4 5 8 12 16 19 19 16 12 8 5
Max (°C) 12 13 15 19 25 30 32 32 28 23 17 13

Further south, near the park of Nea Filadelfia, in the north-central part of the city, 6 km from the centre and 135 metres above sea level, the average temperature ranges from 9 °C in January, to 27 °C in July. Here rainfall is just scarcer, 415 mm per year, being a bit lower in winter.
Nea Filadelfeia average temperatures
Nea Filadelfeia J F M A M J J A S O N D
Min (°C) 5 5 7 10 14 18 21 21 17 13 10 7
Max (°C) 13 14 16 20 26 31 34 33 29 23 18 14

In the National Observatory, at the city centre and not far away from the Acropolis, 100 metres above sea level, the temperature is higher, almost 10 °C in January and 28 °C in July, and is affected by the urban heat island. Here the rainfall amounts to 375 millimetres per year, so it's even lower, and it does not go above the 67 mm of December.
National Observatory average temperatures
National Observ. J F M A M J J A S O N D
Min (°C) 7 7 8 12 16 20 23 23 19 15 11 8
Max (°C) 13 14 16 21 26 31 33 33 29 24 18 14

Going further south, we arrive in the old Ellinikon Airport, located on the coast, in the extreme southern edge of the city. The temperature ranges from 10.3 °C in January, to 27.3 °C in July and August. Rainfall is very similar to that of the centre, and as we have seen it amounts to 365 mm per year.
Ellinikon average temperatures
Ellinikon Airp. J F M A M J J A S O N D
Min (°C) 7 7 8 11 16 20 23 23 20 16 12 9
Max (°C) 14 14 16 19 24 29 32 32 28 23 19 15

The amount of sunshine in Athens is great in summer, when clear skies prevail, while it's not so high from November to February, when the sky is often cloudy, although there may be sunny days also in midwinter.
The sea in Athens is warm enough for swimming from July to September, when the temperature is 24 °C; in June it's still a bit cool, about 21 °C.
Here are the average sea temperatures.
Sea temperature Athens
Athens J F M A M J J A S O N D
Sea (°C) 16 15 15 16 18 21 24 24 24 21 19 17

When to go

The best times to visit Athens are spring and autumn, especially from mid-April to late May, and from mid-September to early October; from mid-October the likelihood of rainfall increases, which sometimes can be violent and concentrated in time. Those who don't suffer from the heat, can also choose the summer.