http://gfp.sd.gov/calendar.aspx
Event Name: Open House and Free Fishing Weekend
Event Location: Statewide
Event City:
Start Date: 05/19/2017 Start Time:
End Date: 05/21/2017 End Time:
Description:
This weekend, enjoy free entrance into all South Dakota state parks and recreation areas. In addition, a number of park will be hosting special events to kick off the summer. Camping fees still apply. This weekend is also Free Fishing Weekend. No fishing licenses are required. Regulations and limits still apply.
I think it has been going on for at least the last few years and probably longer than that.