According to Prof. David Hogg, Monarch specialist at UW-Madison, the first “official” 2016 sighting of Monarch butterflies in Wisconsin was made Thursday May 19 at Pleasant Valley Conservancy.

Here is the report: The first “official” 2016 sighting of Monarch butterflies in
Wisconsin was made yesterday at Pleasant Valley Conservancy, specifically in
the oak savanna habitat on the south facing hillside.  I’ve attached
photos of the adult (female) and one of the eggs she deposited.  We
(Cameron Fullerton and me) watched her flit from milkweed to milkweed, and it
appeared that she laid eggs on a dozen or more stems.  We examined a
number of milkweeds after she left, attempting to target those she had visited
(not easy to do with the large number of milkweed stems in the patch) and were
able to find four of her eggs on stems ranging from 4 to 9 inches tall.

Monarch butterfly on shoot of common milkweed (Asclepias syriaca). David Hogg photo

Monarch egg on milkweed leaf. David Hogg photo