When Mo holds up the card to the secret party in Ray of Light 1, it says “18th” in the top left hand corner. This implies that the party is on the 18th of whichever month.
It is mentioned that it is a Friday, and the only Friday the 18th that makes sense in the show’s timeline is May 18, 2012.
Clare said it had been “about a week” since the greenhouse. The show implies that they all went back to school on the previous Monday, Cam killed himself Tuesday night, and they found him Wednesday.
And wham, Cam’s official date of death is May 8, 2012.