Chambers gives it as either way.
mass2 or Mass noun 1 Christianity in the Roman Catholic and Orthodox Churches: a the Eucharist, a celebration of the Last Supper; b the ceremony in which this occurs. See also High Mass, Low Mass. Compare (in Protestant churches) Communion, Holy Communion, the Lord's Supper. 2 a part of the text of the Roman Catholic liturgy set to music and sung by a choir or congregation • a requiem mass.
ETYMOLOGY: Anglo-Saxon mæsse.