When does the campground open for the season? Are the showers and bathrooms open the same time ?

Ditto

All information and registration is on the Michigan State parks website.

https://www.midnrreservations.com/ Check here.

March 31- October 30, 2019 is their operating dates this year. Yes the bathrooms and showers are open when the park is. This park does allow reservations and I'd strongly recommend them because they are usually full most of the summer.