Take night train "Exp Hamanasu[急行はまなす]"
from Aomori (Not Shin-Aomori/Tohoku shinkansen end stn) to Sapporo.
Some un-reserved seat cars[自由席車](not sleeper type) extend
for Snow fes week usual. (about 120 seats plus/240 seats total)
JR Pass holder no need booking for un-reserved seat cars only.
Other cars need seat booking before dep.
http://urx.nu/ebKthttp://en.wikipedia.org/wiki/Hamanasu_(train)
JR trains(Shinkansen,too) seat ticket sales just one month before
purchasing most JR-East stn in Tokyo possible if not already full.
But basically operate possible or not depends on weather conditions
if by air from Tokyo(HND/NRT) or other area air port,too.
Only Tohoku Shinkansen trains are very strong against the snows.
(closed track, a lot of tunnel sections and sprinklers for melting snows)
BTW,No back to Tokyo after Snow fes?
If use only for oneway no big advantage against regular trains fare 22320Y.
(Tokyo stn to Sapporo stn by Shinkansen and Hamanasu un-reserved)
http://www.hyperdia.com/en/Where to go other days Tokyo and Hokkaido both?