Observing or celebrating a three-day holiday (triduum).
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.)