It's pretty straightforward to get to the events from NYC on Memorial Day weekend.
For Purchase, take the
Metro North from Grand Central (11.55am) (or Harlem 125th St) to White Plains and then
Bee-line Bus #12 (probably the one at 1pm).
To Blue Mountain, take
Metro North to Peekskill and then a 30 minute walk (or a five minute taxi) to the event.
So tell your friends in the city! Four days left for cheap entries.