Even though the park is open year-round, Trail Ridge Road, the park's main east-west thoroughfare, is always closed in winter. It's safe to assume that you will not be able to drive clear across the park from mid-October until Memorial Day, and even in summer it's quite possible that the road will be closed for hours, or even a day or more, by snow.
That's not to say that intrepid travelers can't enjoy the park in winter. All of the park's entrances are open, trails are open to snowshoers and cross-country skiers, and the roads to a number of good viewpoints and trail heads are plowed. There are even a few accessible hiking trails. Skiers with the proper skills and equipment can cross-country ski into the high country, but they need to keep aware of storm and avalanche dangers, and should always check with rangers before setting out. Permits are required for all overnight trips into the backcountry (see "Permits You Can Obtain in Advance," below).
Avoiding The Crowds -- The park is fully accessible for less than half the year, so that's when most people visit. The very busiest time is from mid-June to mid-August -- essentially during school vacation -- so going just before or just after that period is best. Because most people explore the park between 10am and 4pm, you can avoid most of the crowds by going to viewpoints and trails early or late in the day. The light's better for photographs at those times, and you're more apt to see wildlife then, anyway.
Winter is gaining in popularity, even though you can't drive the entire Trail Ridge Road. Although the park can be bitterly cold, this is the quietest time, when you'll most likely see plenty of elk, moose, and other large animals. In addition, the views of stately evergreens on snow-covered mountains set against a sky of azure are unforgettable.
Regardless of when you visit, the best way to avoid crowds is to simply walk away from them on one of the numerous trails.
