From the Spain.I do website:
“The programme brings together theatre performances, concerts, children's entertainment (puppet shows, clowns, magic...), shows, dance, bullfights, craft exhibitions, all kinds of music (from regional folk to pop-rock and electronic sounds at the Independent Music Festival) and many more events. There are also fireworks, the traditional "Giants and Big-heads" parade, sports competitions and exhibitions. The "peñas" (fiesta associations) also play a vital role at the celebrations, which get underway with an opening speech at the Town Hall.
The most important day is 12 October, the feast of La Virgen del Pilar, patron saint of the city. A mass Floral Offering takes place from early that morning: thousands of people from all over the world, dressed in traditional costumes, come to Plaza del Pilar Square to leave flowers before the Virgin. Meanwhile, 13 October sees the Offering of Fruit. And at dusk, the Rosario de Cristal takes place. It is a procession of carriages and glass lanterns that runs through the streets of Zaragoza.”
Sounds like a good time to visit the city with the main activities taking place in the huge plaza by the church.