High season in Brazil lasts from the week before Christmas until Carnaval (which falls sometime in Feb or early Mar, depending on the year). Flights and accommodations are more expensive and more likely to be full during this period. Book well ahead of time for accommodations during New Year's and Carnaval. This is the most fun time to travel -- towns and resorts are bustling as many Brazilians take their summer vacations, the weather's warm, and New Year's and Carnaval are fabulously entertaining. If you want to spend New Year's in Brazil, it's best to arrive after Christmas. The 25th is really a family affair, and most restaurants and shops will be closed.
Brazil's biggest holidays are New Year's (JAN. 1) and Carnaval (March 1-5 / 2014).