Update:

I found an item in my USE inventory that sends me directly to the haunted mansion. Using it sends me to an alternate haunted mansion that has nothing but low level mobs that you can only hit for 5 DMG and they have like 25 HP total. This is where the jack-o-lanterns are. But after leaving this alternate mansion, I'm not sure how to get back since that Item I used up is a one time use item and it's gone now. I'll look around and see if I can figure out how to get back to that alt haunted mansion.

Cassandra can give you the invitation if you don't have it in the inventory.

This is how the event works. The Jack-o-Lanterns are in the invitation mansion, and Cassandra's 'Unfinished Party' gives you a new invitation any time you don't have one (thanks chubby). The Maid in the Haunted House Foyer also sends you to the pumpkin field. We added a test so that re-gift keeps working.