In 1924, the governor of Michigan urged residents to devote May 2 to tree-planting, in the hope "that the date mark the beginning of the greatest tree planting ever conducted in the state."
It wasn't the founding of Arbor Day, though the man who founded it, Julius Sterling Morton, grew up in Monroe County, Michigan and was a student at--but not a graduate from--the University of Michigan. He moved to Nebraska and in 1872, he began the campaign of tree-planting that resulted in today's Arbor Day.
Oh, and the name of the Michigan governor Tree and Bird Day booster?
--April 17, 1924 Ypsilanti Record