Blue moon

A blue moon is a rare astronomical appearance of the moon in its full-moon state, usually with the moon turning blue. In the comic books and the cartoon show, it is used to indicate when a baby Smurf is about to be delivered to the Smurf Village. In the 2011 Smurfs movie, it is a time of celebration for the Smurfs, when Papa Smurf is able to see into the future to foresee what may happen to his little Smurfs. However, it is also the time when a special portal within the Forbidden Falls opens up that transports whoever enters it through time and space, with its other end exiting into a waterfall in Central Park in New York City in the modern age.

In the cartoon show
The blue moon's notable appearance in the cartoon show is in the episode "Once In A Blue Moon", when Baby Smurf first arrived in the Smurf Village. Its other appearance was in the episode "Smurf On The Wild Side", where Papa Smurf retells the origin of "the lost Smurfling", connecting it to the present-day appearance of Wild Smurf.

In the 2011 Smurfs movie
It was during one of these blue moons in the movie's story when Papa Smurf had foreseen his little Smurfs trapped in cages by Gargamel and Clumsy Smurf trying to catch a dragon wand. When Gargamel was accidentally led into the village by Clumsy, he chased the Smurfs out into the forest, causing Papa Smurf and five other Smurfs to head for the waterfall and be transported into the present through that portal. They eventually came across a human named Patrick Winslow who helped them in their search for a "stargazer" so they can determine when the next blue moon occurrence will take place. They also found magic words within a book at Mr. Wong's Mystical Emporium so they could reopen the portal to take them back to their own place in time. During their brief stay with him, the Smurfs had inspired Patrick to come up with a "blue moon" motif as part of his company's advertising campaign, which initially was sent by mistake to his employer Odile in place of the intended campaign design, almost causing Patrick to be fired, but was eventually accepted by Odile when she witnessed the moon literally turning blue through a spell cast upon it by Brainy Smurf.