Optimal period for Western Parks is May-June and September-October — all parks open, hikeable temperatures (Grand Canyon 22-28°C, Yosemite 18-25°C, Yellowstone 15-22°C), reduced crowds vs July-August. Note: Tioga Pass (Yosemite east) generally opens late May-early June by snowpack, Glacier National Park Going-to-the-Sun Road opens late June.
Summer (July-August) is the only period when all parks are accessible, but cumulates downsides: massive crowds (Grand Canyon South Rim 6M visitors/year concentrated in summer, Yellowstone 4M, saturated Yosemite Valley), mandatory reservations (Yosemite Half Dome cables, Glacier National Park entry, Bryce Canyon shuttle), extreme heat (Las Vegas 40-45°C, Grand Canyon South Rim 32-35°C / Bottom 45-50°C, Death Valley 50°C), peak flight and hotel rates.
Winter (November-April) closes most parks. Yellowstone largely closed (only northern Mammoth Hot Springs entrance open, Old Faithful inaccessible by car Nov-April except by guided snowmobile $300-500/day). Yosemite Tioga Pass and Glacier Point closed (November-May). Glacier National Park largely closed (Going-to-the-Sun Road cut October to June). Year-round accessible: Grand Canyon South Rim (magical snowy landscapes, reduced crowds, but -5 to 0 at night), Death Valley (perfect 15-22°C), Zion (8-15°C), Bryce (spectacular under snow but mandatory crampons), Joshua Tree, Saguaro.
Las Vegas is in peak season December to April (conventions, concerts, Super Bowl, NBA, NHL), reasonable hotel rates outside Christmas/New Year and Super Bowl weekend. Avoid July-August (40-45°C, hotel A/C nonetheless freezing).
Read also
- Las Vegas — The Strip, Bellagio, Caesars Palace, Cirque du Soleil: the parks' improbable base camp.
- Grand Canyon — 1,829 m deep, 446 km long, one of the world's most impressive natural landscapes.
- United States — Complete country-continent guide: ESTA, currency, regions.
- California — Combine with Western parks for the complete road trip (Las Vegas → Death Valley → Yosemite → SF).