I researched the best time to visit before my trip almost 4 years ago. Two timeframes seemed best: 2nd half of May/first week of June and 2nd half of August/first week of September. Crowds are significantly less during these two periods (particularly before Memorial Day) and weather is decent. The mosquitos don't usually appear until around the first/second week of June. We visited the 2nd half May/first week of June and the mosquitos let loose in Denali around June 6/7 when we were there. Denali is not completely open til around that time, so if you visit there in May you may be disappointed. I understand that the mosquitos are all but gone by mid-August, although you may encounter snow in early September.