Colombia sits close to the equator, so temperatures don't change dramatically by season — what changes is how much it rains. The country has two dry seasons and two wet seasons, though the timing varies by region.
The main dry season runs from December to March, which is also the busiest time for travellers. The Caribbean coast, Cartagena, and the coffee region are at their most pleasant. Prices are higher at Christmas and New Year, and accommodation books up fast. January and February are particularly good: settled weather, manageable crowds, and Carnaval de Barranquilla falls in February.
A shorter dry season runs from July to August. This is a good time for trekking — Tayrona National Park, Ciudad Perdida, and the highlands see good conditions. Medellín's Feria de las Flores takes place in early August.
The wet seasons — April to June and October to November — bring heavy rainfall, especially on the Pacific coast, which is one of the wettest places on earth year-round. The Andes and Amazon regions also get significant rain during these months. Travel is still very possible: most sights remain open, buses run, and you'll share them with fewer tourists. Trails get muddy but the scenery is greener.
Altitude matters more than season for temperature. Bogotá at 2,600m is cool year-round (typically 7–18°C). Medellín at 1,500m earns its "city of eternal spring" nickname (22–28°C). Cartagena on the Caribbean is hot and humid throughout the year (30–35°C). Pack layers if you're moving between highlands and coast.
Tayrona National Park closes for parts of February (around Semana Santa if it falls then) and in late October for environmental regeneration. Check before you go.