I'm from the Detroit area -- probably close enough.

You should plant them as soon as the soil thaws, which is generally some time in mid to late March. If we have even very cold weather after you plant them, and it freezes up again, it won't really hurt them. They'll take a few weeks to germinate anyway.

I've read that every day you wait after April 1st to plant them resulted in about a 3% decrease in yield in test plots, since Favas are very, very photoperiodic and will bolt to seed when the daylength is long enough, whether or not the plants are big enough or ready.

Favas are fun. Unfortunately, I never get a very large yield out of them -- what usually happens is we have a hot spell just when they bloom, and most of the flowers drop without setting a pod. But, they're very pretty plants, and the tender leaves and shoots can also be used in salads or even as a cooked green.