Smitty,
the colder the air is, the denser the air is. Translation, there is more oxygen per cubic cm or foot. So if there is more oxygen, you must add more fuel. Richen on cold, and lean on hot days.

BTW, dont just arbitrarily richen and lean, do was Dustin said by richening it up first and then slowly lean towards max rpms. Once at max rpm richen it up about 500 rpm. (500 is only good if you have an in-flight mixture control so you can continue to richen throughout flight) Do this with a tachometer and before the first flight of the day. This way it wont matter if it is hot or cold outside, your engine will always be properly set. Although it will make more power on cold days. This should yield good results.