Hi Karen
Within 10 days in June/July I encountered temperature ranging from 4ºC/39ºF to 44ºC/111ºF (and these are the ones I knew of, I think it got colder on the way to O Cebreiro).
You should be OK with the lightweight poncho for the rain (maybe a waterproof cover for the backpack as well) but I suggest you take an extra warm layer in addition to the light jacket. It can get very cold in places, especially in the mountains. I remember arriving in Roncesvalles and being unable to hold the pen to write my details on the form at the albergue - my fingers were numb with the cold.