The term "winter squash" refers to the ability to store/keep the particular squash over the winter due to a hard shell/skin. The squash is slow growing, so you plant earlier in the year so that the maturity date is right before your area's average first frost/freeze date. You want to be picking your last one up until then. Then, since the skins/shell is harder than most, they will store for a very long time over where you can eat them over the winter.