Uruguay has a temperate climate without extreme seasons. The Southern Hemisphere summer runs from December to February, winter from June to August. There is no rainy season as such — rainfall is distributed fairly evenly through the year — but expect changeable weather at any time: Uruguay sits in a transition zone where warm and cold air masses collide, and conditions can shift quickly.
December to February is beach season. Punta del Este, Cabo Polonio, and the Atlantic coast fill up with Argentines and Brazilians on holiday. Prices are at their peak, accommodation books out early, and the coastal towns buzz with energy. Temperatures reach 28–32°C. January and early February are also Carnaval season in Montevideo — one of the best reasons to visit at this time.
March to May is arguably the best time to visit. The beach crowds have gone, temperatures are mild (18–24°C), and prices drop significantly. Montevideo is particularly pleasant in autumn. The Semana Criolla gaucho festival takes place during Easter week.
June to August is winter. Temperatures in Montevideo average around 10–15°C — cool, sometimes grey, but rarely harsh. Some beach resorts close almost entirely. The interior and cities are perfectly pleasant to visit; you'll have Colonia del Sacramento almost to yourself.
September to November is spring: warming temperatures, green countryside, and minimal crowds. A good shoulder season for those who want to see the country without the summer rush.