Map from Google - Galicia

In the autonomous community of Galicia, in north-western Spain, the climate is different from what is normally expected in Spain, in fact it is mild oceanic, with relatively mild and rainy winters, and cool and quite sunny summers. In summer, short heat waves are possible, with highs around or above 30 °C (86 °F). In July 2013, the temperature reached 36 °C (97 °F) in La Coruña, 38 °C (100 °F) in Pontevedra and 39.7 °C (103.5 °F) in Vigo.
The wind blows frequently, and can be stormy in the cold season, so there are several beaches frequented by surfers.
Fog is also common: it can present itsfelf all year round, when there is no wind, even on summer nights.
Snowfalls are only sporadic on the coast (where for example it snowed on the 10th of January 2010); they are still very rare (once or twice a year) in low-lying inland areas, while they become abundant in the mountains during winter.
The rains are usually abundant, since they exceed almost everywhere 1,000 millimetres (40 inches) per year, while they are close to 2,000 mm (79 in) along the west coast and on west facing slopes. However, summer is the driest season, when the rains, though possible also in this season, become quite rare.
In the interior there are mountain ranges, where the climate gets colder with increasing altitude, and in winter heavy snowfalls can occur. There are also skilifts. The highest peak is Peña Trevinca, 2,124 metres (6,968 feet) high, in Serra do Eixe, while the altitude exceeds 1,700 metres (5,600 ft) in Serra de Queixa and Sierra de Ancares.
The coast is indented, and there are coves (called rías) as well as islands. The most important islands are Illa de Arousa (inside the ría of the same name and in the province of Pontevedra), the Cies Islands (off the coast of Vigo, where the famous Praia das Rodas is located), and the Ons Island (slightly to the north, and west of Pontevedra).
The most western point of Galicia and Spain is Cape Touriñán, just north of Cape Finisterre, which is traditionally (and wrongly) considered as the most westerly. This westernmost part of the coast is called Costa da Morte (or Costa de la Muerte in Castilian), because of the several shipwrecks that occurred in the course of history, due to the dangerousness of the coast as well as the frequent storms.
About fifty kilometres (30 miles) south-west of Santiago de Compostela we find a large sandy beach, called Dunas de Corrubedo, protected in a natural park.
In the northeast there's a beach, called Praia das Catedrais or Praia de Augas Santas, where you will find characteristic rocky arches.

Praia das Catedrais

On the northwest coast, in La Coruña the climate is mild in winter, while in summer it remains cool: highs in July and August are just around 22/23 °C (72/73 °F).
Here are the average temperatures.
Average temperatures - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDec
La CorunaJFMAMJJASOND
Min (°C)88910121416161513119
Max (°C)141416161821222322191614
Min (°F)464648505457616159555248
Max (°F)575761616470727372666157

In La Coruna, approximately 1,050 mm (41 in) of rain per year fall; the wettest seasons are autumn and winter, the least rainy is summer: from June to August precipitation drops below 50 mm (2 in) per month, though on average it still rains for 6/7 days per month.
Here is the average precipitation.
Average precipitation - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDecYear
La CorunaJFMAMJJASONDY
Prec. (mm)11590809080453535651351401351045
Prec. (in)4.53.53.13.53.11.81.41.42.65.35.55.341.1
Days15121214117668131515135

Further south, and in the interior, we find Santiago de Compostela, which is a bit colder both because of the distance from the sea and of altitude, and is also rainier, as it receives nearly 1,900 mm (75 in) of rainfall per year.
Lugo, located in the interior and at 450 metres (1,450 ft) of altitude, becomes pretty cold in winter, when night frosts become quite frequent, while in summer, while days are pleasant, nights remain cool.
In Sarria, which is at no great distance from Lugo and practically at the same altitude, and is often chosen as the starting point for the Camino de Santiago (or Way of St. James), the temperatures are similar.
Here are the average temperatures of Lugo.
Average temperatures - Lugo
LugoJanFebMarAprMayJunJulAugSepOctNovDec
LugoJFMAMJJASOND
Min (°C)2235710121210853
Max (°C)111215161922242523181411
Min (°F)363637414550545450464137
Max (°F)525459616672757773645752

In Lugo, the rain pattern is very similar to that of La Coruna: rainfall amounts to 1.115 mm (44 in) per year, with a peak between October and January, when it exceeds 110 mm (4.3 in) per month, and a minimum in summer.
Here is the average precipitation.
Average precipitation - Lugo
LugoJanFebMarAprMayJunJulAugSepOctNovDecYear
LugoJFMAMJJASONDY
Prec. (mm)115908510590553540701401451501115
Prec. (in)4.53.53.34.13.52.21.41.62.85.55.75.943.9
Days14121214137568141515135

In the southwestern part of the coast, in cities such as Vigo and Pontevedra, the climate remains cool enough; inside the rías, in winter the temperature is a bit lower than on the Atlantic coast, while in summer it becomes a little higher.
Here are the average temperatures in Vigo.
Average temperatures - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDec
VigoJFMAMJJASOND
Min (°C)567810131515141186
Max (°C)121316171923242523191512
Min (°F)414345465055595957524643
Max (°F)545561636673757773665954

The rains in the coast exposed to the westerly winds are particularly abundant: in Vigo they reach 1,800 mm (70 in) per year, including more than 200 mm (8 in) per month from October to January; sometimes they reach spectacular values. But even here, in summer fortunately the rains diminish both in intensity and in frequency.
Here is the average rainfall in Vigo.
Average precipitation - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDecYear
VigoJFMAMJJASONDY
Prec. (mm)2101651401551256045451052352502651805
Prec. (in)8.36.55.56.14.92.41.81.84.19.39.810.471.1
Days14121214127558131415131

In the southeastern interior area and at low altitude, the climate becomes almost Mediterranean, and in summer it becomes quite hot: in Orense (or Ourense), 250 metres (800 ft) above sea level, highs in July and August reach 30/31 °C (86/88 °F), although nights remain cool.
Here are the average temperatures.
Average temperatures - Orense
OrenseJanFebMarAprMayJunJulAugSepOctNovDec
OrenseJFMAMJJASOND
Min (°C)33579131515121064
Max (°C)131519202328303128221613
Min (°F)373741454855595954504339
Max (°F)555966687382868882726155

In Orense, precipitation drops below 1,000 mm (40 in) per year, and in summer the rains are quite rare.
Here is the average precipitation.
Average precipitation - Orense
OrenseJanFebMarAprMayJunJulAugSepOctNovDecYear
OrenseJFMAMJJASONDY
Prec. (mm)957060757035202060115110115850
Prec. (in)3.72.82.432.81.40.80.82.44.54.34.533.5
Days119912105346111111102

North-east of Orense, we find the Sil River, which gives birth to the canyon of the same name.

The amount of sunshine in Galicia is good in the summer, when sunny days prevail, especially in the southern part (in July and August there are approximately 8 hours of sunshine per day in La Coruña, 8/9 in Santiago de Compostela, and even 9/10 in Vigo and Pontevedra), while in the other seasons, and especially in late autumn and in winter, the sky is often cloudy, due to the prevalence of Atlantic depressions.

Cies Islands, beach

The sea along the northern coast of Galicia is a bit cold even in summer, as you can see in the following table, regarding the temperature of the sea at La Coruna.
Sea temperature - La Coruna
La CorunaJanFebMarAprMayJunJulAugSepOctNovDec
La CorunaJFMAMJJASOND
Sea (°C)131313131416181918171514
Sea (°F)555555555761646664635957

On the west coast, the ocean temperature is practically the same, as can be seen from the data of Vigo.
Sea temperature - Vigo
VigoJanFebMarAprMayJunJulAugSepOctNovDec
VigoJFMAMJJASOND
Sea (°C)141313141516181918171615
Sea (°F)575555575961646664636159

When to go

The best time to visit Galicia is the summer, from June to August or the first half of September, being the most dry and sunny. July and August are the warmest and less rainy months. However, sometimes it can rain also in midsummer, and some nights can be a bit cool on the coast, and a bit cold in inland areas. Those who don't like the heat, to avoid the albeit rare hot days, can choose the months of May (especially the second half), June and September (especially the first half), knowing though that it could rain more frequently.
In the summer, on sunny days (which are quite frequent, especially in the south-west), you can take a break at the beach and sunbathe, with an air temperature often pleasant and a cool breeze from the ocean, but as mentioned the sea is cold for swimming.