Well, summer is hot but an important fact is that coastal Atlantic cities are much cooler than inland cities and also a bit cooler than mediterrean ones.
From May to the end of September it will very rarely rain and by this I mean once or twice during that period although start of May can sometimes get more.
March-April and October-November offer “cooler” temperatures around 25ºC and is the favoured time for people who want to get out of the gloom of dark northern Europe as normally the temperatures during this period are like German summers for example although at times it is possible to have some weeks when it rains quite a bit, but of course if that is so it will probably be raining even more in Northern Europe.
December – February are as one would expect the coolest (but I deliberately don’t use cold) months of the year with temperatures on rare occasions going down to 5ºC but much more normally sitting between 10-20ºC. It is often sunny and many days can be like a Scottish summer day but these months are of course the most frequent for rain as one would expect.