the a/c does kick on when the tC goes into defrost mode.. it's automatic. back in the day when r-12 (aka freon) was used in a/c systems.. the refridgerant (R-12) was separate from the lubricating oil for the a/c compressor.. so running the a/c from time to time was the only way to keep the seals lubricated and in good condition.. if the a/c wasn't used, the seals would dry up.. and then you had to get a new compressor.
the new systems since around 1994-1995 (depending on car maker) use R-134a.. they have combined the oil with the refridgerant so supposedly it's not needing to be run as much to lubricate the seals, since there is no seperation of oil and refridgerant..
i still run it in the winter.. haven't had any problems with my other cars in doing this over time..
The a/c does have a cut off temp in which it won't work under a certain temperature, let's say.. 32 degrees.. since the a/c get's cold to suck out heat, can't have it get colder than freezing, since that would ruin the compressor and other components, so a temp is picked in which the compressor knows not to activate.. it's safe to try to turn the a/c on.. but the light will come on but no compressor.. most of the new r-134a systems have sensors all around to detect leaks and can shut down the compressor.. unlike the earlier system that the compressor just ran and ruined itself the new ones can have a leak but the expensive part (compressor) can be reused..
hope that helps..