Summer and Winter Resort Post Offices, and South Western Michigan resorts in the 1920s and 30s

 According to the publication "Summer and Winter Resort Post Offices of the United States 1891-1966" by Chester Smith Jr., the Wabaningo Post Office began operation in 1897 and ran continuously until 1938. 

The post office was open from June 15 to September 15 and served the Sylvan Beach Resort and other resorts in the area. 

There were over one hundred resort post offices in Michigan that operated three or four months out of the year during the peak of the resort area in the 1920s and 1930s and 1600 resort post offices across the United States.

Throughout the 1920s and 1930s, the Pere Marquette Railroad provided service from Chicago to West Michigan’s resorts, even running extra trains called the Resort Special in summer months. 

https://www.miplace.org/4a77f3/globalassets/documents/shpo/research-resources/publications/west-michigan-pike-project-report--volume-ii-historic-resource-survey-part-ii.pdf