First time to Spain. When and Where?

I know this is a very open question but we usually go to France or Italy mid-May when the weather is warming up but are considering going away earlier (probably next year) in March or April and Spain is the only reachable country where the weather may be temperate.  The question is, what is the earliest we could go and still enjoy mild and occasionaly hot weather and how far down should we go? Any recommendations?