A permit is necessary for any event (e.g. picnics, weddings, ballgames, gatherings or celebrations) with 20 or more people in attendance. You can apply for all types of permits on the NYC Department of Parks & Recreation website. You can also call 212.408.0226 for Special Events Permits or 212.408.0209 for Athletic Field Permits. http://www.nycgovparks.org/permits