I'm tired of meeting my gardening fantasies by looking at seed catalogues and I'm ready to plant. My dad always said that potatoes could be planted on Good Friday; although in northern Minnesota, the ground was sometimes still frozen on that day. Still, I think some seeds can now be planted in Mankato.
Here in zone 4, the safe first-planting date varies with the spring weather. Most years, the soil is up to 40 degrees by April 15 and it's OK to plant cool-season crops. Several of these vegetables will grow to make grand salads for your Memorial Day picnic.
"Cool season" crops are ones which do better before the heat and humidity of summer hit. Butterhead and head lettuce, leaf lettuce, romaine, and spinach are all greens that grow quickly and easily. Radishes and green onions will mature in time to give variety to your salads. Other salad ingredients that can be planted early include peas (snap, pod, or snow), cabbage, cauliflower, broccoli, and Brussels sprouts. When the early maturing ingredients start to "bolt" or become leggy and bitter, you can turn your attention to the ones that mature later in the summer. Wait until mid- to late-May (after danger of frost) for tomatoes, peppers and other "warm season" veggies.
Plant your salad in a pot or in your flower garden if you don't have a vegetable garden. Forget the idea of tidy rows and plant a small patch. If you are planting a whole garden, use the early crops to mark the rows where later zucchini, tomatoes and peppers will grow.
Especially early in the season, when soil is apt to be more wet, choose a place where you don't have to walk on the soil. You should be able to work the soil without it becoming cloddy, as plant roots need oxygen to grow. The soil should be loose and fine. Plants also need at least six hours of direct sunlight, so forget foundation plantings on the north side of the house or under the backyard shade tree.