The short answer is December to April. The long answer is that it depends what you came to do, and that green season has things the dry one does not.
December to April · dry season
Less rain, more clear mornings, a better chance of seeing the full cone of the volcano. It is also when everything is full and expensive, and when you need to book ahead.
Easter week and the last two weeks of December are the busiest of the year.
May to November · green season
It rains, but not the way people picture it: it rains in the afternoon, hard, for a couple of hours. Mornings are usually fine, which is exactly when you want to do the volcano and the waterfall.
What you gain: half the crowds, lower prices, a forest that is actually green, and waterfalls with far more water. September and October are the wettest.
What changes by month
- Seeing the volcano: dry season, and always in the morning.
- Waterfalls at their best: green season. Three times the water.
- Rafting: green season, for the same reason.
- Price: green season, noticeably cheaper on hotels.
- The hot springs: makes no difference. Sitting in hot water while it rains is one of the best things this place offers.
How many days
Three nights is the honest minimum. With two you do one thing a day and the trip turns into driving. If you are coming from elsewhere in the country, count on three to five hours to get here from almost anywhere.
To see how a full trip comes together, look at our ready-made trips, or the three-day itinerary.