Hi Laurie,
I feel like I need to clarify a bit, just to make sure we are all on the same page. I assumed that you were referring to the spectacular sound and light show known as Los Fuegos de Apostol. This always occurs, for the 3 times I have experienced it, on the 24th not the 25th. The horrible train derailment others have referred to occurred on the 24th not the 25 and los fuegos was cancelled, as I am sure other events in the nearly two week long fiesta were. On the 24th, during the day, there is folk dancing, music and a parade of los gigantes puppets all on Plaza Platerias..south side of cathedral. In my experiences, the 25th is rather somber with the mass as the focus in the day and then concerts by ticket only on Plaza Santa..east side of cathedral. There is a carnival that occurs in the park on the south side of old town all through the fiesta.
Also, los fuegos, in my experience, does begin at about 10 but finishes by 11:30. I always get there by 8:30 for a picnic with pilgrim family and other groups around us! So much fun!
Enjoy!
John