Oceanside is the third-largest city in San Diego County, California, United States. The city is located just south of Camp Pendleton, the busiest military base in the United States. Oceanside is also known for the nature grounds of Mount Ecclesia. This place is the location of the international headquarters of a fraternal and service organization called The Rosicrucian Fellowship. It is also the location of its spiritual healing temple, called The Ecclesia. The California Surf Museum is located in downtown Oceanside. Oceanside is host to the world's 2nd largest Beach Soccer festival, The Southern California Beach Soccer Championships has become an international event and it is usually held in the middle of May at the Oceanside Harbor Beach.