Plural: Vesperisancti, Vesperisanctuses
An evening prayer on a holy day, or a celebrated evening with a prayer (from vesper = evening prayer, and sanctus = holy, saint in Latin). Specifically: Halloween.
—And what about that Vesperisanctus?— reminded Bruno.
—Oh...— smiled the Doctor. —It's not a human. That's the name of the coming holiday. Halloween. The "CLC Eve" in this country.
—All Hallows Eve!— Grandma corrected him obstinately. (Context: Grandma Estela announced in her archaic language that "Vesperisanctus" was coming. A boy named Bruno thought it was a person. Dr∙ Barton starts explaining the holidays forbidden or politically transformed and renamed in their communist country. Source: J∙ Okram - The Mystery of the Rammed Key.)