July and August are peak season — roads are open, facilities are fully staffed, and wildlife is visible, but crowds are significant and accommodation (inside and near the park) books out months in advance. Early morning visits to major attractions are essential; the geysers and hot springs are no less spectacular at 7am and the crowds are a fraction of midday.
May, June, and September are the best months for most visitors. The park is open but less crowded, weather is cooler, and wildlife is excellent. Spring (late May–June) is particularly good for newborn animals in the Lamar Valley. September brings the elk rut — the bulls bugling across the meadows is one of the more memorable wildlife experiences in North America.
Winter (December–March) is a different park entirely. Most roads close to private vehicles and the park is accessed by snowcoach or snowmobile. The thermal features steam dramatically in cold air, wolves are most visible against snow, and the crowds disappear almost entirely. Temperatures can fall well below freezing.
The North Entrance at Gardiner is the only one open year-round. Most other entrances open in April–May depending on snow conditions.