Originally, Halloween was in actual fact celebrated in Ireland and Scotland. Early Jack-o'-lanterns were traditionally made from turnips that were hollowed out and in which children carved faces. On arriving in the USA, emigrants found the native pumpkin, which, easier to carve, replaced the traditional turnip to make the Jack-o'-lanterns, an essential Halloween decoration.
There is much confusion between the traditional pumpkin and the more fibrous less dense Jack-o-lanterns pumpkin. See the article on the pumpkin.