The median last freeze date for most of Milwaukee County usually falls between April 26 and May 2. The last freeze date is the last time in spring the temperature is at or below 32° Fahrenheit until cooler weather returns again in fall.
This range does not pertain to all of Milwaukee County, however, as communities along the lake and further inland are apt to experience different weather patterns than the rest of the area. To see the area where you live, look at this graph showing the median last frost in Wisconsin, courtesy of the National Weather Service.
Note also that this date was compiled by watching weather patterns over 30 years, meaning that the actual last date in a particular year can occur earlier or later.
See also: Median First Freeze Date in Fall