I don't know about Ferreries, but about 3 years ago we went to a fiesta in Es Castell. It was a great day out. The streets were decorated with bunting, there was a fairground, and the main event in the afternoon was a parade with various marching bands, huge figures in national costume, and of course the horse riders dressed smartly, who make the horses rear up on their hind legs, as a demonstration of their riding skills. It is traditional that young men try and touch the rearing horses (can you see that happening in UK?). There are street parties and the drinking of pomada (gin and lemonade) and sometimes the fiestas end with fireworks.

You can see pictures of a fiesta if you type "fiestas menorca" into.... er.... a well known website with a name similar to you cylinder, that shows videos (am I allowed to write that?)

I am sure there will be fields set aside for parking, as there were at Es Castell. Your reps or hotel staff will probably be able to give you directions too.