It is a ridiculous statement to say that there is a 'party season' in Spain. The Spanish are all about everything that defines parties. They love to get together in big groups, drink, eat, dance, and have the best time of their lives. There are festivals, holidays, and celebrations that take place all over Spain throughout the year. But the summer months are the most concentrated form of this. With all of the youth free and in the streets, what better way to keep them entertained than to give them something to celebrate? I would argue that the theory here is that happy children have no reason to cause problems.
It is fair to say that this concentrated part of the year begins in June on with San Juan (Saint John) day, a national celebration to signify an end and a beginning. It is much like how we treat New Years in the US. It is a celebration to get rid of the old and bring on the new. Students, whose school-year has ended, burn notes or tests to show they are done with the year. Some people burn a list of things they want to change in their lives or something they want to forget. In Santander the entire city and the surrounding villages go to the beach where they start bonfires, drink in excess, and bathe in the ocean throughout the night.
Following this nation-wide celebration, the first of the localized festivals begin. Every city, town, village, and settlement has its own unique celebration. Generally it is related to a Patron Saint or significant event of the location.
The celebrations, or Ferias (fares), generally consist of street music, food, and drink, as well as a local custom. One of the more well known festivals is that of the tomato fight near Valencia, which takes place every August, and consists of an entire town throwing old tomatoes at one another. Most festivals last several days, and contain set events for each day, tending to end the festival with the most peculiar of them.
In Tarilonte, the darling village in which my O's grandparents live, the festival is a three day celebration. The first day consists of an honorary lunch for the elders of the tiny town (they hold a winter population of thirty), and large family dinners in individual houses. The second day is the Patron Saint's day, and is centered around attending church, carrying a statue of the Virgin Mary, and ringing the church bells manually. Then musicians set up at night to bring excitement and dancing to the street as everyone dances their shoes off. The third day is the most interesting, and would remind most Americans of Halloween. The events of the day go as follows: everyone dresses in their worst clothes, or cheap costumes, and head to a particular house. Along with the crowd is a walking band that plays exciting music.
Similar festivals are played out in every village on some weekend between June and September. In Santander, it is a two week event that includes street-vendors who sell cheap bite sized snacks with drinks, have fare-rides, and a circus or other similar entertainment.
If you are looking for a time of year to party your way through Spain (which is not at all a hard thing to do) I suggest the summertime. You are guaranteed to find something going on in every region you visit, no matter what days or weeks you have the chance to be here for. I would suggest if you're coming in July or August, that you travel for the Northern regions, because the central and South of Spain are quite warm throughout the summer (reaching triple digits in Fahrenheit almost constantly) while regions like Cantabria, Galicia, and the Basque country are cooler and have rainy spells for all of the Atlantic currents.