Millesimo has a mild upland climate: warm summers, cool nights, and proper winters with occasional snow. The town is at its best from May to October, when the woods are open and the long evenings on the square work in your favour.
Time your visit for July if you want to see the place at full tilt: the Luglio Millesimese festival fills the centre with concerts, theatre, and competitions for most of the month. The white truffle season, late October to December, is also worth catching for the food.