From Longman Dictionary of Contemporary EnglishHigh MassˌHigh ˈMass noun [countable, uncountable] RRCa very formal church ceremony in the Roman Catholic Church
Examples from the Corpus
High Mass• On special feast days a High Mass was sung to mark the occasion.• A High Mass followed in the cathedral.• On the fifteenth of August, after High Mass.• One critic likened it to a titanic High Mass on Mars. 