I don't think Disney releases those official dates until May. I'm sure Josh at EasyWDW is basing his calendar on MK park hours closing at 7pm with no Elec Parade or Wishes. That's a pretty sure sign that's a Party night. But I would go under the assumption that EasyWDW's posting of MNSSHP is unofficial but a pretty safe bet.
We are headed on the Dream Cruise on October 27th thru the 31st and our cruise specialist said that MSSSH is always available on Halloween Night, but a premium price. I know that they have a bunch of great stuff planned for the cruise, including trick or treating, but we are really looking forward to MK Halloween Night!