If your into your evening dining and chill out lounge bars, then aim anywhere between for late May–October, as this is when its warm enough to enjoy sitting outdoors. 

May–October is great for those seeking cultural experiences (i.e., traditional folk dance—Ball Pagès), culinary delights, and nature-based activities like hiking and boating. 

If your more interested in soaking up some rays and laying on a beach, then aim to travel either in June or September when its not so hot, and the beaches are a little less populated. During high season July/ August the beaches can get very over crowded and UV rays are at their highest. 

The winter months are good for green nature, peaceful coastal walks,  and sweet young wines.