/ Dictionary / Index M Muster Day: Meaning and Definition of Find definitions for: Mus'ter Day" Pronunciation: [key] the annual day for enrollment in the militia of all able men aged 18 to 45, according to a law established in 1792 and in effect until after the Civil War. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. muster musth Related Content Daily Word Quiz: cautious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language