Susanville sits at around 4,200 feet in the Honey Lake Valley, which gives it a high desert climate with genuine seasons. Summers are warm and dry with hot afternoons but cool nights — comfortable for hiking and camping. The shoulder months of May, June, September, and October are the most pleasant, with mild temperatures and less crowds at Eagle Lake than the peak of July and August.
Winter arrives seriously. Snow falls regularly from November through March, and the valley can see prolonged cold snaps. The Bizz Johnson Trail becomes a snowshoe and cross-country ski route in good snow years, which makes winter a legitimate season for visitors who come equipped. Lassen Volcanic National Park's higher elevations hold snow well into June, and the park road sometimes does not open until late spring.
If your aim is Eagle Lake fishing and camping, aim for June through September. If you want to catch Bumpass Hell and the Lassen summit hike before the crowds, go in September, when the summer heat is gone and the park road is still open.