We have been fortunate in the Washington, DC area to have a fairly mild fall but the frost must come at some time. When the outdoor temperatures are dipping into the 40's at night for more than two nights in a row, that's the time to bring in your houseplants from the outdoors.
Most houseplants will survive temps down to freezing 32° F or 0°C but it's best to let them get used to having the heat on a little more gradually, so that the move indoors isn't such a drastic change from living outside. Wherever you live, if you haven't brought in your indoor plants from the outdoors, check your night time temperatures.
And if you live in a frost-free zone, just disregard this!