Theodore Roosevelt National Park in August

May through October gives you the full experience without frozen fingers. Late spring brings wildflowers and active wildlife, but also mud if there's been rain. Summer means long days and warm weather, but also crowds during July and August — though 'crowds' here means maybe 20 people on popular trails. September and October deliver the sweet spot: comfortable temperatures, fewer visitors, and prairie grasses turning golden. Wildlife viewing peaks in early morning and late evening year-round. Winter visits are possible but challenging — many roads close and temperatures drop well below zero. If you're chasing bison photography, come in late fall when herds gather in lower elevations. Spring brings baby animals but unpredictable weather that can strand you if roads turn to mud.