Karpenisi has two distinct seasons and is worth visiting in both. Winter (December to March) brings snow to the upper slopes and turns the town into Greece's quietest ski resort — the Velouchi lifts run, the tavernas fill with Athenian families, and the fir forests look Nordic under snow. Accommodation is cheaper and easier to find than in Alpine alternatives, and the atmosphere is genuinely festive without being crowded.
Summer (June to September) draws hikers, rafters, and Greeks escaping the coast's heat. The Karpenisiotis gorge is cool even in August, the Acheloos runs high enough for good rafting through June and July, and the Agrafa villages are at their greenest. Temperatures in Karpenisi itself rarely exceed 30°C — a considerable relief if you are arriving from the island heat.
Spring (April to May) and autumn (October to November) are quieter and can be wet, but the wildflowers in May are exceptional and the autumn colours in the forests are among the best in Greece. Avoid the Assumption feast (15 August) at Proussos Monastery unless you want to join the pilgrimage crowds — or unless you do.