G'day! Down here in Australia we have warm to hot summers and cold, frosty winters ( but no snow..) I plant the peas ( snap sugar & Snow ) in early Fall so they get a good start before the really cold weather hits. They then flower ( up to 7 feet high!) during late winter and are harvested for about 6 weeks through early spring..
Expect this will be the same for you so long as the weather is not freezing during daylight hours.
Expect bucketfulls!

It's not too late. It is a bit on the late side, but still reasonable. If we have a cool summer early on they do really well and yield a bunch, if we have an early, consistent heat up then it tends to be a poor year for them. One never knows what is in store so I would say plant.

Reserve some seeds, or get more seeds for late summer as well. Most of the cool season crops aren't spring crops even though we grow them then, but rather they are fall crops. They perform best when seeds are placed into warm soil and the plants grow in cool air. Mid-late August would be a good time in your area to sow them again.

above statements are right mines only been in ground maybe 3 weeks they are 21/2 in. tall now im only about a hundred miles straight south of chicago so i would get them in very soon or catch them for a fall crop as ou realize our summers are brutal on cool season crops,so we can cheat by planting early and again late,enjoy

I'm just north of Toronto and planted mine a week ago. The only action I have seen is with the Oregon Giant snow peas, and even then they are just starting to peak out. However, our thermometer has been bouncing like a yo-yo with lots of rain lately.

I say go for it! At minimum, peas do a great job improving the tilth of the soil, so even if you don't get peas, you can cut the vines and have a nicely textured soil for a follow-up crop. BONUS - if you fix the seed with an innoculant, AND cut the vines down prior to them getting very large, there will be a significant nitrogen fix for a follow-up crop.

I'm planning on getting mine in this week. It's a little later than I'd like but ok. I've planted as late as mid-May and still gotten a crop.

I planted mine last week, and I've got 91% germination so far (11 out of 12)

I'm w. booberry. I've planted peas on Memorial Day, and had stuff to pick

I would say its worth a shot anytime in April in Chicago. The cold days in March and April tend to slow the growth somewhat. Germination rates for example are very slow in the cold. If someone puts in peas on the March 17 date it may take 2 weeks to germinate. I typically spout them my self especially of they are larger vine types that are longer growing and fewer in number. I plant them as sprouts with a week left in March. You will have missed some of the warm spells but in reality you may only be a few weeks behing because you are just catching some of the warm days. It is critical now because the days ahead are full time growing season.
When the hot weather comes I water frequently and put down a light color newspaper mulch to keep the soil temp down.

As of now my snow pea spouts are 3-4 inches so my sprouting head start worked well. The dwarfs I broadcast took a long time are about an inch. Peas don't mind cold and frost but they grow very slowly in it.

Thanks everybody for the wonderful advice...if only it would stop raining so that I can squeeze in some rows...

Any tips on planting?

My peas are about 5 inches tall now, been in the ground about 3 weeks. I pre-sprouted also. I have mine in 4ft. wide beds and I plant them in 3 inch wide bands zig-zag and about 2 inches apart. I get 5 rows per bed, could probaly fit more. Weather has been crazy but I would go ahead and plant.

I just discovered "Wando" peas yesterday at Home Depot (one more day for their 1/2 off sale, btw...ends today!)
They are for late planting, and are heat tolerant.
I did plant some other peas a week ago, but got these for 'just in case.'

They were in the Ferry Morse display rack.