JoeW has nailed it:
Mt Rubidoux
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Per Wikipedia:
In April 1909, Jacob Riis of New York, a friend of President Theodore Roosevelt, was in town to give an address at the Mission Inn. He suggested holding an Easter sunrise service at the top of the mountain, and on the following Sunday the first non-denominational outdoor Easter Sunrise Service in the United States was held at the top of the mount.
In 1912 an estimated crowd of 3,000 people were present to hear Henry Van Dyke read his poem, God of the Open Air. Each year attendance grew. In 1918 plans for an open-air 10,000-seat amphitheater designed by architect Frederick Heath were considered, but they were never implemented.
The annual service became nationally and internationally known, drawing huge crowds and celebrities, including opera soprano Marcella Craft. In 1915 the Southern Pacific Railroad provided a special service from Los Angeles to Riverside just for the event. The Pacific Electric trolley system also implemented special service from Los Angeles, Corona, Redlands, and San Bernardino. Peak attendance in the 1920s was reported to have exceeded 30,000.
The popularity of the Rubidoux Easter sunrise services spawned many other such services throughout the country, including an annual service at the Hollywood Bowl.[
1913 Mt Rubidoux Easter Sunrise Services:
Wikipedia