I have never attended this gathering, but I have heard that it's great fun, and I might get to go this time.
You will have to do your own research,
Google is your friend, all I know is that it happens two times a year, and the spring ride happens on the first Sunday in May, so that's this coming Sun.
I think the ride starts from
The Red Mouse, in Pine Bluff, WI
It's about a 5 hour ride for me from St Paul, so if I do go, I will probably ride down on Sat, and stay overnight somewhere in the area.
Where to Ride #1 - Slimey Crud Run