Atenas sits at around 700 metres in the Central Valley, which moderates the heat you find on the coasts and keeps humidity low compared to the lowland tropics. Days are typically warm (mid-20s Celsius), nights are cool enough for a light layer, and the weather is pleasant year-round — the town's famous claim to "best climate in the world" is hyperbole, but not by much.
Costa Rica has two seasons: the dry season (verano, roughly December to April) and the rainy season (invierno, May to November). In Atenas the rainy season brings afternoon showers most days but mornings are usually clear. The surrounding coffee hills are at their most vivid green from June to September. The dry season is peak tourist time — more visitors, busier roads, and the landscape turns noticeably brown by March and April.
The Friday farmers market runs year-round. The coffee harvest runs from roughly October through February, which is when finca tours are most active and you can see the full process from picking through milling.