I don't know where you are, but I'm in NE Oklahoma. For me, both times of the year work so I always have young hens starting to lay eggs year round.
Spring is the easiest, because as the weather gets warmer, they can go out sooner without extra heat and they grow faster.
However, I think fall is best "IF" you have a way to keep them warm for a longer time, and a larger area to keep them in. (You don't want to crowd them or they start eating feathers. ) By starting them from now to mid December, you'll have grown chickens by late spring.
I agree with Tony. I only order chicks in the spring, because here our falls are so unpredictable weather-wise. When deciding when to order consider not just where you live, but where the birds are coming from. They'll be making several stops at various post offices along the way and the weather at those stops may be very cold compared to yours.