Bagno Vignoni is at its most atmospheric in winter, particularly from November through February. The thermal pool steams more dramatically in cold air, the valley is quiet, and the experience of soaking in hot sulphurous water while a cold Tuscan wind blows across the hills is one that the shoulder-season crowds never know. The Val d'Orcia in winter light — long, low, golden — is also beautiful.
Spring and autumn are the next best options: the countryside is green or golden, the days are pleasant for walking, and the nearby towns of Pienza, Montalcino, and Montepulciano are not yet overrun. Summer brings heat and more visitors to the wider Val d'Orcia, though Bagno Vignoni itself remains small enough that it never feels overwhelmed. The public baths are open year-round.