triduum
noun
trid·u·um ˈtri-jə-wəm 
ˈtri-dyə-
: a period of three days of prayer usually preceding a Roman Catholic feast
Example Sentences
Recent Examples on the Web Saint Paschal Baylon Roman Catholic Church will hold a triduum July 31 - August 2 in honor of Saint Peter Julian Eymard. 
Maria Shine Stewart, cleveland.com, 21 July 2017
Maria Shine Stewart, cleveland.com, 21 July 2017 Word History
Etymology
Latin, space of three days, from tri- + -duum (akin to dies day) — more at deity
First Known Use
1873, in the meaning defined above