Coimbra sits pretty on a hill over a hill alongside the Mondego River in Portugal's Centro Region. The Romans called it Aeminium. It was capital of Portugal in the High Middle Ages but after losing power it remained a major intellectual center and its university is one of the oldest in Europe.
Coimbra's weather is moderated by it's location close to the Atlantic, and you won't see any extremes in the graphs below. Although mild, winter months tend to be rainy, even more so than Lisbon.
Spring and early Fall are the best times to visit. June is optimal, as the festival season is heating up in Coimbra--the medieval festival is in the middle of the month. August can be hot. Late September through mid-November is also a good bet. If you must go in Summer, reserve your lodging in advance, as this is the major vacation time in Portugal, a destination that's become very popular over the last few years.
The charts below show the historic high and low temperatures in Coimbra, Portugal and the historic precipitation levels.
Serenata Hostel Coimbra (value)