Cam + Derpy day was the beginning of a season. Cam + Derpy day starts on March 2nd, 2012, and ends on December 2nd, 2012.
Cam + Derpy Day was when Des predicted derpy would be found homeless on a street, looking for someone to care for her. The prediction is indeed true, and Cam even confirmed that he claimed Derpy.
He also said he was hiding her so no one would laugh. When he was braver, he'd stop hiding her.