Halloween is Kevin Ryan’s favorite holiday. The Gilt Groupe CEO plans the company Halloween party personally, with a little help from Kym Ganade, head of design and development. Parties get more elaborate each year. The first one, in 2008, was held a Lower East Side bar. Last year’s party cost more than $50,000. Costumes are mandatory. In 2011, Ryan went as a Flash Sale—he wore an overcoat with replicas of items Gilt sells sewn into the lining.