…because I saw it again this morning on a reputable Catholic website, and because recent dialogue here aroused the pedantic grammarian in me (always aroused when challenged and never dormant for long, anyway):
Mass—with a capital “M” is the proper noun name of the liturgical celebration of the Eucharist, and
mass—with a lower-case “m” is the common noun name for any large amount of matter, or any large number of persons, crowded together.
Hear! Hear!
Good to know. I may at times be at fault.