Sisters sits at 975 metres on the eastern slope of the Cascades and gets four pronounced seasons. Summer (June through September) is the peak — warm days in the high 20s°C, cool nights, very low humidity, and weeks of clear sky. The downside is that this is when everyone visits; expect crowds during the major events (rodeo in early June, quilt show on the second Saturday of July, folk festival in September) and book accommodation well in advance.
Autumn (October to early November) is excellent — aspens turn yellow, the high country is briefly walkable before snow, prices drop, and the crowds thin.
Winter (December to March) brings reliable snow and turns Sisters into a ski-base for Hoodoo and a Nordic skiing centre. Town stays open year-round, restaurants and shops reduced hours; some of the higher Cascade highways (notably Highway 242 over McKenzie Pass) close from October until late spring.
Spring (April to May) is the awkward shoulder season — winter snow is melting, the high trails are still buried, but the town is quiet and prices are at their lowest.