Unhappiness Day is an event that took place in the cartoon show episode "Happy Unhappiness Day To You". It is a yearly day of observance for the Smurfs to appreciate the good times by experiencing a whole day of sadness, usually by giving up whatever each Smurf enjoys doing that makes them happy.
Gargamel used a device on this particular day to track down the Smurfs since they are normally happy throughout most of the year, but being unaware of this event, his device ended up leading him to other happy creatures that were more than displeased with the evil wizard disrupting their happiness. When the day ended, Gargamel frustratingly destroyed the device and tossed it into his muck pond when he started hearing the Smurfs being happy again, but the machine was now beyond repair and Gargamel couldn't do anything about it but curse the Smurfs for his action.
Empath: The Luckiest Smurf stories[]
In the EMPATH: The Luckiest Smurf story series, Unhappiness Day actually turns out to be the day that Papa Smurf had brought Empath to Psychelia as an infant, when Papa Smurf thought he had died at the hands of the Psyche Master, and it is also the same day that the last of Papa Smurf's fellow Smurfs had died, leaving him with 97 young Smurfs to take care of. To commemorate such calamities, Papa Smurf had purposely designated that day to be Unhappiness Day, and it has been honored by the Smurfs ever since.