Festa Junina occurs around June 24th or 25th to celebrate the nativity of St. John the Baptist. It's worth pointing out that we're not Catholic and that Brazil is making large shifts towards Evangelicalism. But, it is still Brazil, so the country celebrates the Saints as national holidays. I studied the Bible quite a bit while in college - English literature connects back to it so often that it's necessary. But admittedly, I don't know a lot about the Saints. Long story short, St. John prophesied the coming of a messiah, i.e. Jesus. To be honest, I've heard nothing about St. John during all the Festa Junina celebrations.
The party is really similar to a country fair in the States. It takes place in a large tent-type set up. At the little expat's school, it was in the gymnasium. The parties I've been to in the States occurred either outside with no cover, or outside with a tarp-tent set up (minus the walls). Men and boys dress up as "hillbillies" (or peasants/farmers) with checkered shirts, jeans with patches, and straw hats. Women wear "hillbilly" dresses (usually checkered again) and will wear their hair in pigtails or braids. People will paint on freckles and mustaches. The party features lots of dancing and music - a lot of the music is folk songs or children's songs - and of course fireworks. Brazilians will light off fireworks at 9:30am to celebrate a chicken laying an egg. Really, they set them off for everything. In our state, Minas Gerais, "Caipira" culture is displayed during Festa Junina. Caipira is used similarly to "red neck." It can be used as an insulting or self-identifying word. (It's also the root of Caipirinha.) Sometimes Festa Junina will feature a mock wedding with a silly acted out story of the bride getting pregnant and her father forcing the offender to marry her - a shotgun wedding. It's supposed to convey a little moral story to the viewers, but it's usually more funny than educational.
At Maicon's school, the students and teachers wore traditional Festa Junina clothes and each class performed a dance in a roped off area in the middle of the gym. Maicon's class of 4 and 5 year olds featured a dance that consisted basically of jumping up and down in circles. But it was adorable and they all had a really good time.
Festa Junina is a little different in every state, but here is a great link to a comprehensive guide. You can read all about the different dances, food, drinks, songs, etc. Although, it is in Portuguese. (Bust out that Google Translate app!)