night train reservations

How far in advance can I reserve a private compartment on the night train? Also, just want to confirm that the night trains between Tokyo and Sapporo take reservations in both directions?

Seat ticket sales starts from one month before. No sales out of Japan.
Don't know your travel date if Fri,Sat,before day of National holiday,
during long school holiday(Spring,Summer and Winter) seasons hard especially.

Night train names are "Cassiopeia" and "Hokutosei" by JR-East/JR Hokkaido.
http://www.jreast.co.jp/cassiopeia/
http://www.jreast.co.jp/cassiopeia/accom/index.html
http://www.jreast.co.jp/cassiopeia/hokutosei/index.html
from/to Ueno(Not Tokyo) - Sapporo stn via Sendai and Muroran side both.

Cassiopeia is seasonal train not operate every night and No solo(single bed) room.
Hokutosei is regular train operate every night both way solo(single bed) room with
but solo(single) room sold out faster usual. No seat cars both all sleeper(sleeping) cars.

Japan Rail Pass not covered both all fare need extras.(depend on seat class)
http://www.japanrailpass.net/eng/en004.html