I have a septic odor that only happens when the weather turns cold. This is throughout my house. Any ideas on how to find it and why it happens only during cold weather?

Where does your vent stack terminate? If the odor is everywhere I suspect the vent stack. If it is more concentrated near a toilet then it could be a bad wax ring on the toilet. When the heat is on your home may have a slight negative air pressure due to the combustion in the heating unit,

The vent stack runs through my basement, to my bedroom closet, to my attic, and out through the roof.

Is the odor concentrated more in one room than the others? Any trapped fixtures like a sloop sink or floor drain that could have evaporated out their water seal?