/trij"ooh euhm, trid"yooh-/ , n. Rom. Cath. Ch.
a series of special religious observances over a three-day period, in preparation for a great feast.
[ 1880-85; triduum period of three days, equiv. to tri- TRI- + -duum diwom, akin to dies day (long i perh. after postridie on the following day) ]