The best time to climb Denali, or Mt McKinley, as it was also previously known, starts around late April or early May. From there, the peak climbing season extends until the period when the snow starts getting too soft for climbing, which is around late June or early July. That doesn't leave a very big window for climbers who wish to enjoy the most dependable weather conditions, though the fact that only some 1,200 people attempt to summit in any given year helps to keep the slopes relatively uncrowded at all times.
For inexperienced mountaineers, the best time to climb Denali is arguably never, as the task is best left to the highly skilled. The tallest peak in North America, Denali tops out at an impressive 20,237 feet above sea level. That makes it more than a mile higher than any other mountain that can be found in every other U.S. state. Mt Whitney, which is the tallest mountain in the contiguous 48, rises to an elevation of 14,494 feet above sea level. Due to Denali's elevation, the weather on the mountain is often harsh, with the average temperatures at the high camp falling somewhere between 20 to 40 degrees below zero. And that's during the summer season!
Interestingly enough, while Denali remains one of the most difficult mountains to climb in the entire world, inexperienced climbers are known to attempt an ascent to the summit. Some even make it, which is often attributed to favorable weather conditions. Part of the reason why inexperienced climbers decide to take Denali on in the first place is the fact that the renowned West Buttress route has a reputation for being relatively easy. This being said, even the most experienced of climbers have lost their lives on the route, so nobody should take it lightly by any means.
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Every year, approximately 1,200 people attempt to climb Denali. About half of them succeed. The most unfortunate lose their lives. This often has to do with severe weather moving in, which can happen with very little warning. On the injury front, frostbite is something that many Denali climbers have suffered, and every now and then, rescue operations are required for various reasons. Thankfully, there is an emergency clinic on the mountain. It sits at an elevation of 14,200 feet and keeps a rescue helicopter on standby. This helps to reduce major injuries and deaths among climbers.
Because of its degree of difficulty, Denali often serves as a training ground of sorts for mountaineers who are preparing to climb even loftier peaks, such as Mt Everest. The difficulty that is associated with climbing Denali also leads the majority of inexperienced climbers to arrange a guided expedition to the top. These guided climbs aren't cheap, and spending the better part of a month on the mountain is pretty much the norm regardless of how you choose to do things.
Late April to early July is definitely the best time to climb Denali, due to the availability of flights. These flights are those that depart from the town of Talkeetna and take passengers to the Denali base camp. The elevation of the base camp is around 7,200 feet, thus making it a good place to start assimilating to the high altitude. Other camps at loftier elevations provide other places to rest along the way. Talkeetna is about a two-hour drive from the nearest commercial airport in Anchorage. Many climbers choose to rely on shuttle service to get from the Anchorage Airport to Talkeetna. Flying and taking a train are other options.
As a side note, the various climbing routes that lead to the top of Denali are all difficult, though some are more difficult than others. Only the most experienced of climbers take on the Cassin Ridge trail, for example, and the West Rib route is not for the unseasoned climber either. As for the "easier" routes, the Karstens Ridge/Muldrow Glacier is similar to the West Buttress route in terms of difficulty. It is also known for having less traffic than the West Buttress route, though the longer approach from Wonder Lake tacks on an extra week's time on average. Either way, climbers should prepare themselves for the climb up however possible. Numerous climbing guides that focus on the lofty peak are out there, and that's a good place to start.