What an interesting holiday. In that case, I can surmise a few reasons why costumes might come into play.

( There's no shortage, seeing as almost any festival or occasion would encourage some style of dress for the occasion. Perhaps people want to blend in with those spirits, good and bad, who are coming to visit this side of the veil—or maybe it's just about embracing the atmosphere. There will be more opportunities for Tighnari to question it either way. As much as he's curious, for now, the matter at hand is that of the costume this young lady has picked out. Glancing back to where she's set it aside, he offers a nod, more for himself than his conversation partner. )

I'd say it's fitting, if a bit simple. You can always incorporate pieces from another one to embellish it too.