Sunday morning was dark (obviously) and rainy (unfortunately) before dawn, so we skipped going back to the Cathedral (also because a million people, literally) and drove to about the halfway point of the pilgrimage and watched everyone streaming by when we were a little more awake. We happened to be near the church along the route where the car for the Virgin stops briefly.
A lot of these photos from the Romeria are of danzas. Some groups walk, some dance the entire way, some carry drums along, others roll them, some wear very elaborate clothing. A ethnomusicologist friend of ours told us after last year's Romeria that these groups are mestizos using dancing created after the Spanish conquest that were specifically designed to be part of Catholic worship in Mexico.
Most of the route is along Camacho which is a divided road. People walked along one side the danzas and other groups were on the other side, with the Virgin following them at the end. When we were watching near the Basilica last year, we only saw the danzas coming in. The walkers went in a different way.
A huge number of people participate in this. We had people streaming by completely filling a 6-lane highway, for two hours. Then there are all the people watching, the city employees from both Guadalajara and Zapopan, volunteers helping to make everything run smoothly, people who work for the churches, and vendors galore. I can easily believe that 1 million people are involved in some way (I've seen crazy high estimates of 2-3 million people, but that seems unbelievable. But one million I can believe).
I think we lucked out in how we saw this even both years. I liked going to the Basilica and being on that end very much, and I'd probably recommend that option for people going just once, but I loved going to the Cathedral the night before when it wasn't so crowded (and you have to get there really early and hope you find parking). Watching along the route was good too because I felt like we were part of it a little more.
Also, I liked the man handing out oranges to everyone.