Planting guides often say something like "plant after average last frost" or "plant after danger of frost has passed". But what does that mean? Average last frost is the date on which, in 50% of previous years, the last frost had already occurred. The "last frost date" mentioned in planting instructions is typically the 90% last frost date.
I have found weatherspark to be a useful tool for visualizing the risk involved. It plots the mean, 25th percentile, 10th percentile, and record lows.
I also set up an alert using IFTTT to email me if the forecasted low for tomorrow is below freezing, so I can think about covering plants or disconnecting hoses.