On the 6th of July the running of the bulls festival, which is known as San Fermin, has its opening ceremonies. At noon, a rocket is shot from the balcony of their city hall. Everyone dresses in white from head to toe with a red sash around their waist and a 'panuello' or scarf around their neck. You can buy the sash and scarf for 6 euros and you can also find cheap white clothes since this time of year they have TONS of 'rebajas,' or sales. I would suggest not investing in too expensive of clothing because it is going to get doused in sangria, wine, sweat, dirt and god knows what else. But make sure to dress the part because you don't want to be labeled a 'girri,' or tourist.
For the chupinazo, everyone packs into the city. They estimated that this year there were one million people in the city for san fermin. It is impossible to move really. You typically just get tossed to and fro with the masses. It is like being stuck in a sea of drunk people during a storm, complete with the spray of sea water (or in this case sangria) in your face. There is pandamonium everywhere. It is easy to get separated from people and even harder to find them again since everyone is wearing the same thing!
If you would like an idea of what the chupinazo is like, then see the following link: