It's called Festa Junina or Festa de São João, and from it's religious origins, it celebrates the nativity of St. John the Baptist, although it has a background on pagan religions' festivities of the summer soltice.
Nowadays the general festivity doesn't really carries the religious meaning, exept maybe for the catholic church. In fact, in Curitiba they make this ginormous cake with around 35 thousand pieces. There is a little medal in 8 thousand of these pieces, and it's said that the women who can find the medal in their piece are soon to be married.
Other then that, we just have a funny party, with traditional goodies such as pumpkin sweets, sweet popcorn, canjica (a sweet dish made of white corn), caricatural country dressing, games, typical decoration and sometimes even some traditional dancing. It's actually really funny.
Here are some exemples of the decoration
We call the little flags "bandeirolas" (they were portrayed in many of Alfredo Volpi's works, but that's a conversation for another post). In the middle pic you can see the ballons that used to be released at night. This was forbidden since they'd cause fires when they landed, so now they're just part of the decoration.
Food stuff: Canjica, pumpkin heart shaped deserts and pé-de-moleque.
And....I'm having quite a hard time finding pics I can post here, so...yeah, I'll let you lol at me a bit
So this is me all happy in my Festa Junina clothes, at the age of God only knows :D
Hope you enjoyed the post. See you soon!