Residents of 21 Southeastern Massachusetts communities and those who live within 1,500 feet of the border of those communities are urged to stay indoors, close all windows, and shut off air conditioners during a scheduled aerial pesticide spraying Saturday night, the Massachusetts Department of Public Health said Friday. The spraying, from 8:15 p.m. through 2 a.m., was ordered after health officials detected Eastern Equine Encephalitis samples in mosquitoes in the region, the department said. The communities being sprayed can be found on the Department of Public Health’s website, www.mass.gov/dph. A first round of spraying was scheduled to take place Friday evening from 8:15 p.m. through 2 a.m, the department said.