Potosí sits at 4,090 metres and has a cold, high-altitude climate year-round. Even in summer the nights are freezing, and the days rarely get above 15°C. The dry season (May to October) is the better time to visit — clear skies, cold nights, and no rain. The wet season (November to March) brings afternoon thunderstorms and occasional snow, making mine tours and the Khari Khari walk less appealing.
The altitude is the main consideration. Give yourself at least a day to acclimatise before attempting strenuous activity — mine tours, long walks, and even climbing stairs will feel harder than expected. Drink coca tea (freely available), move slowly, and avoid alcohol on the first day.