The high mountain air of Sucre is brisk but generally mild throughout much of the year. It is never hot and seldom very cold. In fact the weather is pretty consistent year round in Sucre, with daytime temperatures in the high 60s to low 70s, and significantly cooler in the evenings.
Winter, which lasts from June until September is actually mild, with little to no rainfall and no snow. It seldom reaches freezing, but the mountains can make it feel a bit cooler, and it isn’t uncommon for it drop 30 degrees or more after the sun goes down during the height of winter.
The spring and early summer, which is from late September to early December (with the seasons flipped as this is South America), is generally the warmest time of the year. Summers tend to be cool, and also the wettest season. January and February get the most rainfall.
Thus the springtime is typically the best time to visit, as there is only a slight chance of rain, mild weather and plenty of sunshine. But whenever you go, plan on bringing at least a warm jacket for the chilly evenings.