Rainbow Day is a special day that the Smurfs celebrate which appears in the cartoon show episode "How To Smurf A Rainbow". It usually begins with a raincloud that appears over the Smurf Forest on that day, which then dissipates and leaves behind a rainbow. The Smurfs gather around the Great Oak Tree, which is decorated with streamers of various colors, to see the rainbow appear in the sky, and then return to the village to have a Rainbow Day party.
In the episode, which is Baby Smurf's first Rainbow Day, Mother Nature had to attend to a volcano and thus left her rainbow recipe book in Papa Smurf's hands so he could create the rainbow for Rainbow Day. Unfortunately, Papa Smurf wasn't able to follow the recipe exactly, and thus an incomplete rainbow formed, which became rather upsetting to Baby Smurf. Michael the leprechaun, who showed up at the Great Oak to place his pot of gold at the end of the rainbow, was unable to do so because of the incomplete rainbow. Fortunately, Michael was inspired to let the Smurfs use the gold from the pot to complete the rainbow, from which he will collect it at the other end. Gargamel, who was out in the forest looking for the legendary pot of gold left by the leprechaun, only found an empty pot and was not able to get the gold which was placed inside the rainbow in time before the rainbow itself disappeared.