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.
Other busy times of the year include Easter week and the months of July, when Brazilian schools and universities take their winter break, and August, when most Europeans and North Americans visit during the summer vacation. This is probably the worst time of year to travel; prices go up significantly, and except for in the north and parts of the Northeast, the weather can be iffy and downright chilly from Rio de Janeiro southward. One year in Rio, I suffered through 4 straight weeks of rain, and temperatures as low as 5 to 10 Celsius (40s-50s Fahrenheit) are not unheard of in the south. If you want to take advantage of the best deals and still have good weather, consider visiting Brazil in September or October. The spring weather means warm days in São Paulo, Iguaçu, and Rio, and tropical heat everywhere else; in the Amazon and the Pantanal, you'll be there just before the wet season starts. As an added bonus, in Rio you'll be able to attend some of the samba school rehearsals as they get ready for Carnaval (yes, they start 4 months early). Another good period for a visit is after Carnaval (early to mid-Mar, depending on the dates) through May, when you can take advantage of low-season prices, particularly in hotels, while still enjoying good weather.
As Brazil lies in the Southern Hemisphere, its seasons are the exact opposite of what Northern Hemisphere residents are used to: summer is December through March and winter June through September. Within the country the climate varies considerably from region to region. In most of Brazil the summers are very hot. Temperatures can rise to 43°C (110°F) with high humidity. The Northeast (from Salvador north) is warm year-round, often with a pleasant breeze coming off the ocean. Temperatures hover between the upper 20s to mid-30s Celsius (low 80s and mid-90s Fahrenheit). The winter months (June-July) are slightly wetter, but even then the amount of rain is limited -- a quick shower that cools things down briefly before giving way to more sunshine.
As befits a rainforest, the Amazon is also hot and humid year-round, with temperatures hovering around the mid- to high 30s Celsius (mid-90s to low 100s Fahrenheit). The dry season lasts from June to December and is often called "summer" by the locals as it is hot and sunny. As the rivers recede, beaches and islands reappear. The wet season typically runs from December to May and is referred to as "winter." The humidity is higher in the rainy season, building up over the course of the day to produce a heavy downfall almost every afternoon. Even then, however, mornings and early afternoons can be clear and sunny.
The Pantanal is very hot in the rainy season, with temperatures climbing over the low 40s Celsius mark (100°F). Most of the rain falls December through March. The driest time of the year is May through October. In these winter months things cool down considerably, though nighttime temperatures will seldom drop below 20°C (68°F). Rio has very hot and humid summers -- 38°C (100°F) and 98% humidity are not uncommon. Rio winters are quite mild, with nighttime temperatures dropping as low as 19°C (66°F), and daytime temperatures climbing to the pleasant and sunny 30°C (86°F). Cariocas themselves find this lack of heat appalling, and will often throw on a coat or heavy sweater when the temperature drops below 21°C (70°F). In their defense, I should note that most houses and apartments are completely without heat, and many restaurants and stores lack windows or doors, so it can feel quite cool.
São Paulo has a similar climate to Rio's, hot in the summer and mild in winter. As São Paulo sits atop a plateau at approximately 700m (2,300 ft.) of elevation it can sometimes get downright chilly, with daytime lows June through September sometimes reaching 12°C (54°F). South of São Paulo, things get even colder in the winter. In Florianópolis, many restaurants and even some hotels and pousadas shut down for the winter season. Also, in the mountain resort of Petrópolis and the historic towns of Ouro Preto and Tiradentes, it often gets cold enough to see your breath (5°C/41°F) in the fall and winter, and Brazilians will travel here to experience winter.
The following holidays are observed in Brazil: New Year's Day (Jan 1); Carnaval (Mar 5-8, 2011, Feb 18-21, 2012); Easter (Apr 4, 2010, and Apr 24, 2011); Tiradentes Day (Apr 21); Labor Day (May 1); Corpus Christi (June 3, 2010, and June 23, 2011); Independence Day (Sept 7); Our Lady of Apparition (Oct 12); All Souls' Day (Nov 2); Proclamation of the Republic (Nov 15); and Christmas Day (Dec 25). On these days banks, schools, and government institutions will be closed, and some stores may be closed as well.
Brazil's biggest holidays are New Year's and Carnaval). Easter is also a big celebration in a number of towns around the country, particularly in the historic towns of Minas Gerais and Novo Jerusalem outside Recife. Reservations are recommended for those planning to attend these events.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.