The Romeria or Fiesta of the Virgin of Zapopan is on October 12 and goes from the Cathedral in Guadalajara to the Basilica of Zapopan. A long procession goes ahead of the image of the Virgin of Zapopan and a huge number of people in the area participate. It seems that the conventional wisdom among expats is to just avoid the area because of the crowds and traffic, but I think the Romeria is well worth going to. Here are a few ideas for attending. See here for photos and more explanations.
The main pilgrimage begins early on the morning of October 12. We never were committed enough to get up in time to see the beginning of the procession at the Cathedral, but you'd need to get there around 5 AM to see everything. This would be one of the most crowded times. Instead, we went to the Cathedral on the night of the 11th to see all the preparations and to talk to people who were participating in any way. I wanted to see what was going on there and the night before worked out really well. There were still a lot of people, but it was possible to find park and to see what was going on.
A huge number of people walk from the Cathedral to the Basilica. They go north on Alcade, then northwest on Camacho. The dancers and other groups are on one side of the divided road and most of the regular walkers are on the other side. We arrived at the intersection with Castillo at about 8 AM when the procession had been going for a while, but it's so long that there was still plenty to see. I liked watching along the route this year. It was easy to find parking.
The church of San Ignacio de Loyola is along the route and is apparently the only place where the car carrying the image stops for a few minutes, so that can be an interesting place to watch from, but it's obviously more crowded.
The first people begin arriving at the Basilica around 9ish. We watched from there last year, near the arch. We didn't see the pilgrims from there because they had split off before, but we were right at the end where the danzas finished. We also could see the image when it arrived. We arrived at 8:30 last year and that was plenty early for the Basilica. So unless you want to get up really early, I'd recommend either watching somewhere along the route or going to the Basilica.
After the image arrives, the area around the Basilica is full of people and they do Mass outdoors. There are many, many people around the Basilica all day, and it's busy the next day too with people coming to see the image and danzas everywhere. We went last night (October 13), the day after, and there were still long lines to get into the Basilica and plenty to see. My husband thought it was one of the best times to see the danzas.
Last year we went on the morning of the 14th when it was a little quieter and easier to get into the Basilica. They were beginning to take down the barricades and things were settling down.
This truly is a huge and wonderful event. If you're in Guadalajara in mid-October, be sure to see some of it.