Here is a link from the National Climate Data Center (NCDC) that gives last freeze dates...
Here is how to interpret the data using the example below.:
At the station located 2 miles North-northeast of Adrian, there is a:
90 percent chance of freezing temperatures (32F or below) after April 20th
50 percent chance of freezing temperatures after May 4th
10 percent chance after May 18th
For the station 5 miles Northeast of Allegan, these dates are April 22nd, May 10th, and May 27th, respectively.