Tankslave
Depends on what you want to do. Snowboarding, probly late November...it's not too could yet and it has been dumping snow early November since I've been here.
Fishing, for salmon, summer time. Strict fishing regs and non-resident EXPENSIVE
Sightseeing/hiking/camping...summer (late May-mid Sept). It starts snowing again in late October. There are basically 2 seasons, winter and summer. Winter is dark most of the day (daylight -0930-1700) Summer daylight, extreme 0330-2330.
As Maxalmon says, there is plenty to do here all year round. I would like to go on that train ride. The Knik river produces some huge salmon. I've been to some glaciers but not Portage yet. Glaciers are awesome. Last year I picked wild blueberries off the side of a mountain...it was pretty steep.
There are some pretty nice microbreweries here in Anchorage. Great pizza and Amber beers at The Moose's Tooth restaraunt. Also a great place for calamari and other fine foods and beer is The Glacier Brewhouse in downtown Anchorage.
Have a good trip.