When you're writing about a specific fair, you should use its proper name the first time it is mentioned. In this case, you'd say something like, "The Seville Fair is held every year in Seville, Spain." And then every time you mention the word fair again in your story, you can leave out Seville and just call it "the fair." So, for example, you could then say: "On the last day of the fair we have fireworks."
"At the fair, women dress in traditional costumes all "flamenca dresses." They are long and colorful with flares." (I think you might mean ruffles, rather than flares.) People dance 'sevillanas,' a traditional dance, and drink sherry. All this takes place in 'casetas,' a special kind of canvas tent with a counter and bar inside, set up especially for the fair. Another part of the fair features an amusement park with all types of rides. (Carnival might be better than amusement park in this example--carnival is a temporary ride setup, whereas an amusement park is usually a permanent venue.)