Advice from the greenhouse lady here...Put them in the ground asap and water them in. When they freeze down and dry up, clip off the dead and mulch them lightly with straw or leaves.
Next spring when danger of frost is passed, remove the mulch and separate the plants if you wish. Pinch out the tops to make them bushy, fertilize and continue to pinch any blooms out until July 4th.
I know a lot of people don't follow these steps, but this is from the book, and the best way to have really beautiful mums next fall.
Note: If you leave the plants outside in the pots, the roots will get too cold and dry, and the plant will more than likely die.
If you take the plants inside, they will not get to go through their necessary dormant cycle.
If you don't want to plant the mums, cut them back to the soil, water well, put the pots in a protected spot outside (in a corner, or between some shrubs and the house), and mulch them heavily with straw or leaves. Don't put any kind of waterproof tarp over them, as they'll need the moisture from the rain and snow. A piece of landscape fabric or burlap, anchored down to keep the mulch from blowing away is fine. The main thing is to protect them from the wind and keep them from drying out. Moist frozen soil doesn't fall below 32, which helps to protect the roots.
We get by with a little help from our friends