When are train tickets available ?

Next year, I would like to travel on the Trans-Siberian railway, from Moscow to Krasnoyarsk, and from there to Beijing. After a few days in the Chinese capital I would then like to take the rail to Lhasa. Could you please tell me how long before travelling I can buy these tickets. The more time there is, the easier it would be to arrange the necessary Russian, Mongolian and Chinese visas. Thank you.

Daniel Parsons.