"-Samhain festival is now completed for every girl."
Does this mean their reactions to the player's progress is now fully implemented for the first event?
Yes, during the festival event each girl will have a reaction based on your performance in the Trial of Mind.
when is that?
The festival is right after the Trial of Mind. On November 1st
oof, mighta passed over that
Yes, if you have had already progressed past November 1 in the previous version, you will miss this event. It is not necessary for the game progression though. So if you don't want to start over just to see it - you don have to.