"Happy Unhappiness Day To You" was an episode that appeared in Season 5 of the Smurfs cartoon show.
As two Smurfs ring the village bell to summon every Smurf around Papa Smurf's speaking mushroom, the village leader tells the gathered Smurfs that the following day is Unhappiness Day, the saddest day in the Smurf calendar. Sassette is curious as to what the day is about and so asks Papa Smurf, who then tells her it is a day that Smurfs set aside to be unhappy so that they can appreciate how wonderful happiness is all the rest of the year. To that end, Papa Smurf lays down the ground rules for Unhappiness Day: (1) every Smurf must get up on the wrong side of the bed, (2) no smurfberries for breakfast, and most importantly (3) each Smurf must give up what they like doing the best: Harmony's horn playing, Vanity's gazing at himself in the mirror, Farmer's farming, Grouchy's doing nothing but griping, and Papa Smurf's work on his experiments. The Smurfs then decide to have as much fun and happiness as they can have before Unhappiness Day comes.
Meanwhile, in Gargamel's castle, the evil wizard is ranting and raving about the Smurfs having fun and being happy out in the forest, and in his ranting causes Azrael to be knocked off his perch on the window sill and into a barrel of water. Gargamel then decides to build himself a happiness detector that will help him find Smurfs since they are happy all the time, according to his thinking, and with it he can scoop up Smurfs by the dozens.
The morning of Unhappiness Day comes, and as two Smurfs ring the bell signaling the dawn of that day, the Smurflings wake up realizing what day it is and they're already unhappy with having to go through it. Out in the forest, Gargamel is using his completed happiness detector to find Smurfs when he hears an alarm inside it going off, meaning that it has found happiness somewhere. Gargamel then follows the detector, assuming that it is leading him to the Smurfs, when he finds out that it has led him to Bigmouth taking a bath in the river. Soon Gargamel finds himself chased by a very angry Bigmouth, and though he and Azrael escape him by hiding within a hollowed tree, the evil wizard is still adamant about going to find the Smurfs.
Back in the village, Greedy is in his kitchen with something he's cooked up, ready to take a taste of it, when Brainy pops out of the cupboard, catching Greedy in the act of trying to make himself happy. Brainy tries to excuse his behavior by saying he's doing Greedy a favor by making sure he stays true to the purpose of Unhappiness Day, but Greedy decides to "return the favor" by sealing Brainy up inside his cupboard.
Out in the forest, Gargamel is again on the trail of finding Smurfs when his happiness detector goes off a second time, alerting him to the possibility that he may find Smurfs this time. Instead, the detector leads him to a bear happily feeding himself with honey from a bees' nest, and neither the bear nor the bees are happy when Gargamel stumbles into them. The evil wizard and his cat run for their lives as they are chased by the angry bear and the bees.
Near the village, the Smurflings are secretly doing happy things out of sight of any Smurf -- until Brainy pops out from behind a bush, again catching Smurfs in the act. He's ready to let Papa Smurf know what they are doing, but Papa Smurf shows up and finds out that Brainy is enjoying sneaking up on Smurfs on Unhappiness Day. Brainy admits to feeling sad that he's been found out, and so joins Papa Smurf and the Smurflings in being unhappy at the count of three.
Meanwhile, Gargamel is still at it with trying to find the Smurfs when his happiness detector goes off yet again, this time leading the wizard to another place where he hopes to find the Smurfs. Instead, though, the detector leads him to a cave where two dragons are passionately kissing each other, and the disrupted dragons literally give Gargamel a "farewell toast"!
Back in the village, Brainy hears Smurfette singing the Smurf song nearby and heads in her direction. She is secretly watering the flowers in her garden, not wishing to see them go thirsty that day, when Brainy catches her in the act of being happy while watching her from the top of a ladder. Smurfette gives Brainy a well-deserved tongue lashing when Brainy falls down from the ladder, hurting himself in his landing. He cries out for medical attention, and soon two paramedic Smurfs come by with a stretcher.
Out in the forest, Hogatha the evil witch cackles as she has caught herself a frog, one that she has changed from a handsome prince that she hopes to marry, when Gargamel's happiness detector leads him straight in her direction. Thinking that he's found the Smurfs this time, Gargamel ends up bumping into Hogatha, knocking the jar out of her hand and setting her prince-turned-frog captive free. The witch is greatly incensed over what Gargamel has done and sends both him and Azrael running for their lives, saying that they can't escape from her.
In Papa Smurf's house, the village leader looks at a bandaged Brainy who now feels as terrible as he looks, and says that now that the sun is setting, they can bring Unhappiness Day to a close. Papa Smurf has Harmony tell the bell ringers to signal the day's end, which he does so "with pleasure". In Gargamel's castle, though, the evil wizard is so unhappy with his happiness detector that he goes to great lengths to destroy the machine and give it a proper burial in his muck pond. But soon he hears the Smurfs being happy again, and so dives into his muck pond to resurrect the machine, but it is now ruined beyond repair, making him curse the Smurfs for it.
Back in the village, Papa Smurf congratulates all the Smurfs for how they handled themselves during Unhappiness Day, including the Smurflings. Brainy tries to pat himself on the back for helping Greedy not have a bite to eat all day, which makes Greedy so miserable and angry at Brainy that he gives the bespectacled Smurf a well-deserved boot to the village limits. Brainy just mutters to himself that he thought that Unhappiness Day was all over!