The pumpkins spawn in all the fields ie Britain, Mooonglow, Yew, Haven.
If it is like the past years, you must get the pumpkins off the server or destroy them to get them to respawn.
If you have a necro the Horde Familiar comes in real handy, other wise get a beetle or packy and load them up and bring them to another server, ie Luna.
Don't just put them on the side of the field, they will decay at a slow rate.