Hailey Bieber dressed to the nines to celebrate her 24th birthday—even though the fashion fête never left home. Yesterday, the model took to Instagram to share a preview of her birthday look, which consisted of a low, slicked-back bun; mini gold hoop earrings; a slinky, black cut-out gown courtesy of Yves Saint Laurent by Anthony Vaccarello; and one serious leg slit.