...in addition, browse older messages here in the "Weather" section of the forum. Reading older messages (and looking at the date they were posted), might also give you some idea regarding the weather at that time.
I walked from St Jean in April 2 years ago. The weather was perfect for the first two weeks, glorious sunshine but not too hot to make it uncomfortable. It then rained continuously for several days and snowed in the mountains. Beautiful fine weather from Burgos to Astorga, but sticky mud in patches that made for hard going. Sometimes there was a very cold wind blowing off the snow clad peaks. I was glad of a windproof jacket, ski gloves and a woollen scarf which I wrapped around my head. It snowed between Rabanaul and El Acebo. Then glorious hot weather for a few days, then rain continuously for the last few days in Galicia.