Ideally you want your soil to be nice and warm for when you plant your pumpkin plants into the garden. Cold soils will stunt the growth of the roots and can make it difficult for the plants to get at nutrients. probably the best way to warm the soil is heating coils, but I don't have any of those. So what I do is put down plastic over the planting area a couple of weeks prior to planting to help get the soil warmed up. It will also keep the planting area relatively dry so when it comes time to put the plants in the ground you don't get caught with a muddy patch of soil to put your plants into.
Clear plastic works best I think to warm the soil. It won't get overly hot like black plastic which could start killing the biology in the soil, but should keep it at a nice moderate temperature.