Halfwit, who changed his name from plain Freddie after becoming a housemate, turned 24 on Sunday, the day of the Summer Solstice.
Big Brother said the Solstice was often associated with hippies and gave the housemates some costumes to wear, to Halfwit's delight.
He was thrilled with his tie-dye outfit but later told Angel: "This is probably the weirdest birthday because it's all being broadcast."
Angel revealed Halfwit was not the first housemate to have a birthday while in the house but she had asked for hers to be kept a secret.
"I don't like birthdays," she said. "Doesn't make me happy. I asked Big Brother do not celebrate my birthday."
She refused to say the date but added: "It is already gone."
Halfwit was told to prepare a 15-minute presentation about himself entitled The Life, Times and Thoughts of Halfwit the First, for the other housemates.
"I was born with a brain with very little understanding of social signals," he began.
"If I had been aware of social signals, it would have crushed me. I've always been a bit different. I didn't notice people calling me stupid."