The ideal time to plant is at least six weeks before ground-freezing frost is expected in your area so that the bulb has time to root. So how do you know when a hard frost is expected? Here are two resources:
1) DavesGarden.com has a handy freeze/frost lookup tool. Just enter your zip code.
2) Your local Cooperative Extension office will be able to assist you. Use the USDA Partners and Extension Map to find the office nearest you.
But, what if December rolls around and you still haven't planted the bulbs? As long as you can dig a hole in the ground, go ahead and plant them. They'll probably rot if you leave them in the bag. At least they'll have a chance of blooming if you put them in the ground.