First depends on if we are talking about legal casinos as defined by the US government or casinos being first operated on their own by the Indians.
There were many Indian "casinos" operating in several different states before the government got involved.
The Indian Gaming Act of 1988 established Indian casinos as being "legal" for the first time.
On the Michigan information, Triple 777 Club opened 7-84, becoming Kewadin Bay Casino in 1988, Kewadin Bay Casino in 1989. The Ojibwa "Casino" at Baraga opened in a garage with one blackjack table 7-85, with the real casino building opening 11-85. Vegas Kewadin at Sault St. Marie also opened 11-85.
I would have to search all my notebooks to find when Indian casinos leagal or not opeened in each of the other states. Also it would first have to be decided what constitutes a "casino". I, for one, do not count bingo only as a casino.
Hope this helps a little. Let me know if you have any more questions. Janice O'Neal