In October, the growth of warm season grasses like Saint Augustine, Bermuda, Centipede, and Zoysia begin to slow down. Now is a bad time to do things that disrupt the turf, such as filling, aerification, or thatching.
By mid-December, most warm season grasses will be dormant to some extent. Dormancy is important for the survival of grasses when they could be exposed to freezes. This would be a bad time to apply fertilizers high in nitrotgen. Nitrogen will stimulate lucious fall growth, which would make the grass susceptible to cold injury in the winter.