Decepticon Day was the name of a celebration made by humans in honor of the heroic Transformers, the Decepticons, for all the good they did for the people of Earth, such as protecting them from the evil Autobots.
No, you read that right.
The Transformers cartoon
After Shawn Berger revealed to the citizens of Central City incriminating footage that showed that the Decepticons were the "good guys" who were being exploited by vile Autobot propaganda, the in-progress Autobot Day was canceled and later re-worked into Decepticon Day.
A more martial affair than its Autobot counterpart, Decepticon Day involved a parade with the marching Decepticons in front of their adoring fans, led by Shawn Berger.
After the humans learned the truth, the Decepticons were hated once more and defeated by the Autobots whose good names were restored and Shawn was permanently thrown off his high horse.