How early can I purchase tickets to Beijing South from Cangzhou West?

Hello, I am travelling from Cangzhou West to Beijing South on October 2nd (meeting up with my parents during Golden Week), I understand this is a very busy time, and I have had a number of people give me different answers as to when I can earliest book in.
I dislike being unprepared and/or a trouble to my hosts, so I plan on buying ahead as is suggested.
Thankyou in advance for your help and information :)