The Pilar Festival is held every year around the second week of October in Zaragoza. The festival features a huge flower offering on October 12th in which thousands of flowers are piled into a gigantic pyramid in the center square next to a huge basilica. It's a great photo op.; here's a (not so good) picture I took of the flower pyramid from last year's festival:
The festival is also accompanied by a number of cultural performances, parades, concerts, bull fights, &c.
Edit: In terms of weather, it depends what you're looking for. I've been all around Spain in different seasons, but when I was there last fall we primarily toured in the North (Madrid → Zaragoza → Atxondo → San Sebastian/Bilbao → Elciego/Rioja → Segovia → Madrid). About 10% of the days there was rain, and it would sometimes drop down to the mid-teens (Celsius) at night, but it was generally quite pleasant weather. I wore a light jacket most of the time. The South would have been noticeably warmer.