…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.