After Date in Spring
Temperature 10% 33% 50% 67% 90%
32˚F or below 10-May 30-Apr 25-Apr 20-Apr 10-Apr
24˚F or below 9-Apr 1-Apr 27-Mar 22-Mar 14-Mar
16˚F or below 13-Mar 3-Mar 25-Feb 19-Feb 9-Feb
Before Date in Fall
Temperature 10% 33% 50% 67% 90%
32˚F or below 2-Oct 11-Oct 16-Oct 21-Oct 30-Oct
24˚F or below 1-Nov 9-Nov 14-Nov 19-Nov 27-Nov
16˚F or below 28-Nov 6-Dec 10-Dec 14-Dec 22-Dec
Data from: WJ Moyer, ESSA-Weather, UMD, 1970
How to read this chart
There is a 10% chance you will see a freezing temperature after May 10 (1 in 10 years), and a 90% chance of seeing a freeze after April 10 (almost every year) 
Similarly, you have a 10% chance of seeing freezing weather before Oct 2 (1 out of 10 years), and you can expect to see freezing temperatures by Oct 30 (9 out of 10 years). 
Please Remember, These data are from the Research and Education Center. Your dates may vary due to other factors, such as proximity to water and elevation.