@Donny in 2018 I walked from Lisbon starting in October and reached Viana de Costelo on 1 November, taking the coastal route out of Porto. It had been fine up til then, but from 4 November on there was constant cold hard rain, all the way to Santiago. A friend flew to meet me in Vigo, and each morning we would get up, don our rain gear, walk in rain for several hours, then repair to a warm bar. Once we were in Santiago we bought broaches featuring rain, to celebrate our survival!
That was in 2018. Each year is different. I liked it enough that I went back this year, and we had glorious weather (but it was in April). Good luck!