I’m not Wiccan but I find it endlessly fascinating to learn about the old, old traditions, how the early Christian church adapted/co-opted them, and how they’ve evolved into what we have today. (See also: Saturnalia, Eostre, etc.)
For Halloween, we can thank the Celtic pagans for the idea of spirits being close, the focus on death and blood and bones, and even trick-or-treating–and you can learn all about it in the Wikipedia article.
Turns out the originators of the holiday had some of the OG creepy costume ideas, too. I present the Láir Bhán (similar to the Mari Lywd, for any readers of The Dark is Rising series):
In parts of southern Ireland during the 19th century, the guisers included a hobby horse known as the Láir Bhán (white mare). A man covered in a white sheet and carrying a decorated horse skull (representing the Láir Bhán) would lead a group of youths, blowing on cow horns, from farm to farm. At each they recited verses, some of which “savoured strongly of paganism”, and the farmer was expected to donate food. If the farmer donated food he could expect good fortune from the ‘Muck Olla’; not doing so would bring misfortune.
(Image from here)