"Be careful of humans this time of cycle; because this is when they perform a ritual they call 'Halloween.' They place images of human skeletal structures, women with green skin, humans biting into other humans' necks, and other frightening images." -- Anon Guest
Some Humans celebrate mortality. Others celebrate horror and the unknown. In both circumstances, part of the celebration involves dressing up and a certain volume of depictions of Human skeletons. The cultural significance of these have been through more than five hundred years and mashed through the sieve of misunderstanding by the fork of ignorance. It should be no small surprise that such are easily confused by outsiders.
The key difference is that those celebrating Dio de Los Muertes never go door to door demanding sweets and threatening pranks. Mix in overlapping calendars depending entirely on how out of sync a Human colony became over the intervening centuries, and you get passages of time marked as Human Deception Festival on the Galactic Calendar.
Some species just never got the hang of theatrical makeup, or facepaint. It's not their fault that they're easily confused by Humans overdoing that particular aspect of the celebrations. Most of them entirely miss the thing with the pumpkins or turnips because one creature's specific holiday decoration is another's casual decor. The net effect is further exacerbated by attempts to render the various holidays safe for Havenworlders.