I am afraid your question is as vague as it comes, with the necessary differences, is like if I was asking what to pack to go to the US, north and south. The orography of the Iberian peninsula makes for great weather differences depending on where you visit. Not necessarily temperature-wise, south in Sept can have a pleasant 25ºC and 18ºC in the north... but, if you go, to a more mountainous area (say for example Vall d'Aran, by the Pyrenees in Catalonia), the average temp will be around 15ºC and so on. However, couple that with humidity and also chill factor, and here you have "large" differences. It doesn't certainly feel the same 18ºC with a 75%+ of humidity -typical on certain days in many coastal cities- than the same temperature in a more dry area (ie. Castille). Another factor is rain, April-May and Sept-Oct are "wet" months and rain is present in many areas, especially on the northern coast (Basque Country, Cantabria, etc). And again, if you come from a cold zone, under 20ºC can feel summer to you, but if you come from a warm one, you'll probably need a light jacket in the evening!