Sam technically celebrates Halloween alone, though many animals of the forest join him.
He is on the mountain for almost one full year living alone, but he is joined by Bando and his father for Christmas. Sam's father also stays with him until the new year begins. The only holiday Sam celebrates alone is Halloween.
On Halloween, Sam is feeling lonely. He decides to throw a party, and he puts out food for the neighborhood animals.
On Halloween, Sam places food outside of his tree and invites the wild animals to eat the food. This is his way of celebrating Halloween - by offering treats to his neighbors in the forest.
Sam is not completely alone, however, as his falcon, Frightful, is with him throughout the great majority of his time in the forest.