Guimarães is comfortable from late April to October. Summers are warm without being suffocating — daytime highs around 27°C in July and August — and the nights cool enough to want a light jacket. Winters are mild but wet; expect rain for at least part of any visit between November and March.
The biggest annual event is the Festas Gualterianas in early August, three days of medieval-themed processions, fireworks, and concerts that fill Santiago Square and the historic centre. São Nicolau in late November/early December is a smaller, student-led festival rooted in the University of Minho. If you want the city to yourself, come on a weekday in May or September.