When to Go
You'll find plenty to do year-round in Anchorage, though most visitors (particularly first-timers) might be happiest from late May through early September when the days are longer—up to 19 hours, 21 minutes during the summer solstice—and the temperatures warmer. If you choose one of the shoulder seasons, go with fall. There's less chance of rain, the snow has not yet arrived on trails except in the highest mountain passes, and there's an excellent chance for warm, sunny days, cool nights, and dazzling color changes in the trees and tundra. But if you choose fall, pack a few extra layers as a just-in-case for early snow (or be prepared to visit one of Anchorage's many gear shops): the city's earliest measurable dose of snow fell on September 20, 1947.
Of the snow-free months, May is typically the driest, while August and September are the wettest. July is the warmest month, with an average temperature of 58.4°F; May is the coolest at 46.6°F. But don't be fooled by statistics. Late-May temperatures can exceed 70°F, and "hot" July and August days sometimes break 80°F. Of course, rainy low-pressure systems from the Gulf of Alaska can skulk in at any time, bringing wet and cool weather.
With such vagaries, do as the locals do: come prepared to go with the flow. That means packing light rain jackets and layers as well as tank tops and sunblock, and allowing for some flexibility with your plans.