First of all: you do not go to Brittany for two weeks of sunbathing on a crowded beach in the summer. If that is what you love, you are better of choosing a destination on the Mediterranean Sea.
Just like most countries in Europe, Brittany has four seasons. All four worthwhile to travel to Brittany for. The climate in Brittany is characterized by a mild maritime climate. This mild climate is due to the warm Gulf Stream around Brittany. The Channel Islands of Jersey and Guernsey even have an almost subtropical climate. Because of the warm Gulf Stream, the winters in Brittany are rarely cold and it almost never freezes.
Brittany summers are pleasant with an average temperature around 22 to 25 degrees Celcius. Brittany gets more than 2200 hours of sun per year. In comparison, Toulouse in southern France has an average of 2000 hours of sun. Brittany's summers usually begins in mid-June and continues until mid-October.
In winter the temperature is generally around 6 to 10 ° Celsius. The winter is also the period when severe storms occur in Brittany. It is a spectacular sight to see the huge waves slamming on the rocks.
It is believed that Brittany knows a lot of rain. This is not at all true. Of course it occasionally rains in Brittany, but also striking is that the amount of rainfall measured in Brittany is not much different from the South of France. The midlands (the Monts d’ Arree) get the most rain in Brittany.
It must be said that the Breton climate can change rapidly in one day. In one day you can wake up in the mist, go sunbathing on the beach in the afternoon to wear a sweater during your evening meal.