I watch the weather and when there aren't any lows below about 40 in the forecast, that's when I plant out. Usually 2-3 weekend in May. To get started earlier, you could use something like row covers or hoop covers or something like that.

I am about an hour north of Madison, WI and I get my plants out full time usually in late May. I would say a week earlier for you. But like tsheets says, around early May, start watching the forcast. If it is calling for a week plus of night temps of 40 or above, then I would plant. even if you wait until later in the month, don't plant out until you know you will be out of the woods temp wise unless you are prepared to mess with covering everything. Actually, the plants will likely survive anything down to freezing but it will put them in a dormant mode for a while after.
I will be planting my seeds in the middle of February for a late May transplanting outside. I guess that would put you a week or so out if you go by my schedule. However, it is up to you. It winds up being a trade off between dealing with ever growing plants and providing them adequate light before you get them outside.

If you started your seeds today, this is what you will likely be looking at in early May for plant size. I planted them on January 23rd two years ago and by May 10th, this is how big they were. The black containers are about 6" diameter for a size reference.
Bruce

Remember, calendar dates, like "last-frost-date", are averages, and are not cast in stone. Every year is different. Let the conditions be your guide. Air temperature is very important, but so is soil temperature (assuming you're planting in the ground of course). It should be at least 60F, if not warmer. You can purchase a soil thermometer inexpensively, if you don't have one. Transplanting too early when the ground is too cold can negatively impact your plants and may cause them to not perform/produce well the entire season. One way to help warm the ground faster is by using black plastic mulch.

Just my 2c. Good luck and have a great grow.

Rick