Aurora viewing season in Alaska begins in late August and lasts through early April. But to combine aurora viewing with peak winter activities (like dog sledding, snowmobiling, and skiing), we recommend you visit between December and March.
If your focus is viewing the northern lights, we recommend you dedicate at least 3 days to the activity. Most winter vacations are short, around 4-5 days. This is plenty of time to see the lights (if the weather cooperates) and to enjoy some winter fun during the day. But if you want to stay longer, there's plenty to do!