We are planning a trip to Spain next spring. In order to have good weather, how early can we go? Also, what part of Spain should we concentrate on; we only have 1 week.

You're planning a trip in spring so as late as possible in "spring" ensures the best weather.

What do you want to see in Spain? Seville is a fabulous place, but then again equally wonderful is the Basque region, come back with more specifics please.

the south of Spain gets very hotmuch sooner than the north - we were in Seville in April and it was already in the 80's. San Sevastian, on the other hand - is very cool - except in the middle of the summer. So you will need to have some idea of your destinations before you decide on an exact time.

What are your main interests?

Have a look at the Fodor's Spain section, see what interests you the most - and then come back with some specific questions.

If good weather is a big factor, perhaps you should concentrate on Andalusia. Perhaps three days in Seville, a couple of days in Granada and some time in one of the Mediterranean resort areas, such as Nerja or Marbella.

I agree with Marty---Andalusia !
Easter week[Semana Santa] is spectacular in much of Spain, but the processions in Seville are renowned. We loved the procession at Nerja one Easter Sunday at noon. It was very emotional for me. I would not go earlier than Easter and I would head south. The centro plateau is still cold that time of year.

The Easter week parade including the Lady of Macarena in Seville is worth being a part of. Seville is a fabulous city, lots to see/do but small enough to get around easily. In the opposite direction - Barcelona is fabulous. Gaudi, Picasso, Las Ramblas. I would spend the entire week there.

Ive just returned from 3 weeks in spain, and enjoyed the larger cities far more than the smaller ones. Barcelona is amazing...there's so much to see there in terms of art, history, and just the fact that it's an amazing city built on the water. Beautiful. You could spend a week there, easily, there's so much to see.
Madrid was also great, although far different from Barcelona, seeing that there is no water. The parks and museums in Madrid are unbelievable.

As for the smaller cities, San Sebastian is a gem. A romantically beautiful city, where on a clear day you can actually see to France.

Seville was nice too, but small, like San Sebastian. In both of these cities I was a bit stir crazy after about 3 days.

Also, however, keep in mind that the farther south you are, the warmer it'll be that time of year.

Barcelona, however, if i had to recommend one, would be my choice. It's one of my favorite CITIES in Europe.