"What started at the turn of the century as an effort to gain a day of recognition for the significant contributions the first Americans made to the establishment and growth of the U.S., has resulted in a whole month being designated for that purpose."
-Library of Congress webpage
"The first American Indian Day in a state was declared on the second Saturday in May 1916 by the governor of New York. Several states celebrate the fourth Friday in September. In Illinois, for example, legislators enacted such a day in 1919. Presently, several states have designated Columbus Day as Native American Day, but it continues to be a day we observe without any recognition as a national legal holiday."
From the Native American Heritage Month website - https://nativeamericanheritagemonth.gov/about/