Spook-A-Smurf Eve is a day of observation that the Smurfs celebrate in the cartoon show episode "Monster Smurfs". It is analogous to the human day of observation called Halloween, and probably celebrated sometime around that day due to the fact that in the cartoon show universe Jokey's birthday happens to fall on the day of Halloween. On that day Smurfs dress themselves up in costumes to the effect of making themselves scary enough to win Mother Nature's prize for the scariest costume. It is also celebrated with a party which includes games and roasting smurfmallows around the fire and listening to ghost stories around nightfall.
In the cartoon episode during the holiday, Tailor had come up with an Azrael costume that he and Clumsy wore that was so real it had fooled the other Smurfs into thinking the real Azrael had found their village. However, Brainy was using Papa Smurf's magic dust in his laboratory to change himself into a real monster so that he is sure to win the prize. The other Smurfs found out about this and called Brainy out for cheating by using magic. Brainy in turn offered to change all the Smurfs into monsters, with the promise that he will able to change them back into real Smurfs.
After their transformation into monsters, the Smurfs went out into the forest to find Tailor and Clumsy and found them, not knowing that a tree stump that had come to life by lightning and made into a monster has joined them. The monster saw Smurfette in her monster form and fell in love with her at first sight during the party. When Papa Smurf returned from Mother Nature's house to find that his little Smurfs were changed into monsters, he realized Brainy was behind this and ordered all the Smurfs to go to his laboratory to be changed back. Smurfette was eager to find out who the tree stump monster really is and had him go before her in line, but Papa Smurf's magic dust revealed that the monster was a real monster as he grabbed Smurfette after she changed back into a Smurf and took her into the forest. The other Smurfs thought Smurfette would be in danger from the monster, but as it turned out, the monster was more afraid of Smurfette because he really didn't want to be a monster. After the tree stump monster came to Smurfette's rescue by scaring away the real Azrael, the monster joined the Smurfs back in the village and waited until morning for Mother Nature to present her prize, which she gave to Tailor and Clumsy for their Azrael costume. Then, realizing that it was her lightning that changed the tree stump into a monster, Mother Nature summoned the lightning to change the monster into a happy flourishing tree that the Smurfs will take good care of.