Zone 4:
Average dates the last frost - 1 May to 30 May
Average dates the first frost - 1 September to 30 September

Those are just averages and every year there seems to be something different and unexpected. But up here, I'm confident in putting things out on or around Memorial Day. Would probably go out a little sooner if I were in the Cities or southern MN.

The seed packets generally tell you when to start them indoors. Unless you've got grow lights and a good indoor seed starting set up, starting them too early in a sunny window often results in damp off or seedlings getting too weak and spindly. At least that's been my past experience. Now I occasionally start a few annuals in the Jiffy peat plugs mid May and have had better luck.

I start my peppers, eggplant, and tomatoes around the first week in April and put them under lights once they have germinated. I've started my Parsley and dichondra and they have germinated. Read the seed packet, it will tell you how many weeks to start them before transplanting outdoors.

Hot weather crops, like peppers and tomatoes, plant around Memorial day in the Twin Cities. People plant a week earlier, which might be okay, but you don't gain anything in the end.

Cool crops like broccoli, cauliflwer, cabbage,etc. plant out May 1-10.