Ever since that incident when I found that my head could drop off (>>454453), I fell in love with that very old and forgotten creature, the dullahan/headless horse — and whenever I slept I felt like my head had vanished. That is the basis for my Nightmare Night (Halloween) costume: many think that it's about house-shopping and asking for candies and apples in water but those are the youngest leaves on an old tree of festivals for remembering the dead; All Saints' Day (1 November) follows and is itself followed by All Souls' Day (2 November) and the three days together constitute Allhallowtide. (For some reason the syllables get squished together and my pronunciation becomes "Valhalla-tide"…)
The dress is quite thin and yet deep, a reference to my double passions of science and mythology. On my neck, below the point where my head is severed, lies a purple ribbon with a white line on it for a noose, suggesting that I was hanged (which I thought I had been down those corridors in the National University of Equestria's chemistry department when it was merely broken glassware). The dress follows the same pattern with white lines running downwards, but there are two gaps in each of those lines when viewed from one side (four in total). With those three resulting lines somepony could see them as the head, neck and rest of body — what would be left after a dullahan strikes — or a ribcage; then there is a chemical interpretation, the many struts in a Vigreux column separating the "head" compounds from the main "body" of a mixture or the cleavage of bonds in fragmentation reactions. That is everything about the costume, but note that all these meanings exist because of many ponies who lived and died long ago, which is what Nightmare Night/Halloween was born from: a desire to preserve that which sprang from what was no longer there.