Plural: triduums, tridua
A time interval of three days. Specifically, a holiday or festivity that lasts three days, such as Christmas (Christmas Eve, Christmas Day, and Boxing Day), Paschal Triduum (Easter), or the autumn triduum (Halloween, All Saints' / Hallows' Day, and All Souls' Day).
But there are other three-day holidays, such as Paschal or Easter Triduum. An analogical one is, I think, from about the eighth century, if I recall correctly. It is still commonly called the Triduum by some people today. That one starts in a couple of days. As per the "Tridumations", I guess that's a local folksy term from some village in Estela's family line. Satisfied? (Source: J∙ Okram - The Mystery of the Rammed Key.)