Mega Halloween International Ball at the Westin St. Francis Hotel – One of San Francisco’s biggest Halloween events and costume party. This party features ten DJs, Go-go dancers, stilt walkers and a costume contest with prizes. Oct. 31 9 pm – 2 am. Absolutely an AMAZING Line Up! Wear your sexy-scary best as costumes are required for entry to this MEGA must attend HALLOWEEN PARTY on Union Square. Act fast. This Halloween event sells out every year.