I started on Aug 23rd last year. The precise date was dictated by the availability of a bed in Albergue Border. The first day up the mountain it was very hot and very humid so I was very pleased to have a "half-way" stop. The next day it was very windy, which was a great improvement on the humidity. A few days later people had heavy rain! The conditions in the Pyrenees are unpredictable. I do remember it being very hot at night too.
It was truly beautiful with fabulous sunsets and sunrises. My accommodation was booked until Pamplona and then a day or two ahead until Sarria, where it was much busier so I booked a few more days ahead. There were plenty of people on the Way but never problem numbers. My hope was to come between the high numbers of Spanish (July and August) and the high numbers of non-Spanish (first two weeks in September). Hard to tell if that worked but I certainly wasn't in a huge wave of people.
If you're walking all the way to Finisterre you will arrive with a high chance of very pleasant weather if you start in August. (However it will be more crowded.)
If it's hot then start early, take breaks in shade, finish early, wear a hat and drink lots of water.
Buen Camino.