Any springtime gatherings in the Upper Midwest?

As you can see from the map below, we've pretty much got the Southwestern states "checked off" and now Liz and I are looking for a way to check off some more states, specifically Nebraska, North and South Dakota, Minnesota, Wisconsin and maybe Iowa.
So are there any gatherings planned, especially for Springtime in 2015? Looking at past years I can see there's a gathering in Sheboygan, WI over Memorial day, but any others?
We'd rather go early in the year, just because going across Nebraska or South Dakota in the heat of August doesn't seem like it would be much fun. But it seems like April or May should be perfect in that area, right?
Thanks in advance for any tips!
So are there any gatherings planned, especially for Springtime in 2015? Looking at past years I can see there's a gathering in Sheboygan, WI over Memorial day, but any others?
We'd rather go early in the year, just because going across Nebraska or South Dakota in the heat of August doesn't seem like it would be much fun. But it seems like April or May should be perfect in that area, right?
Thanks in advance for any tips!