triduum
A period of three days (especially in Roman Catholic liturgy).
I’m much more comfortable on Maundy Thursday, the beginning of the Triduum, the holiest three days in the Christian calendar.
Where it comes from
Borrowed from Latin trīduum, from trēs (“three”) + diēs (“day”).
Adapted from the Wiktionary entry triduum (CC BY-SA 4.0).