Stirling shares the Scottish central belt climate: cool and often wet, with the best weather typically in May, June, and September. July and August bring the most visitors but also some of the warmest weather; the castle esplanade and the Wallace Monument are very busy on summer weekends. Spring and early autumn offer a better balance of manageable crowds and reasonable weather for outdoor sightseeing.
The Hogmanay and Burns Night period (late December to late January) gives the town a particular atmosphere, with local celebrations and the nearby Scottish Political Archive at the University hosting events. Summer festivals in the town and at the castle are worth checking if you are visiting July–August.
Rain is possible at any time of year. The castle and the main indoor sights are fully accessible in wet weather; the walk to the Wallace Monument and the Cambuskenneth Abbey is considerably less pleasant. Pack layers and waterproofs regardless of season.