how long advance I need to book guesthouse and train ticket from Xian?

You sand you come to Beijing on Oct. 1 and stay until the 6th. Then you ask how early you need to book guest house and train ticket from Xian. I am totally confused what you really want to ask.

Did you mean that you will go to Beijing from Xian? If so, you can book the ticket for Oct. 1 on the internet on August 3. As for your accommodation, you can book around a month in advance. However, if you already set your schedule, then I think you can book the guesthouse as early as you can. You know, October 1 to 6 are Chinese National Day Holiday. There will be a lot of tourists in Beijing. If you book your accommodation too late, I am afraid that you may not find a satisfying guesthouse.