Why is the ninth month called September?
c.1300, "to display, reveal, appear," from Old French mostrer "appear, show, reveal," also in a military sense (10c., Modern French montrer), from Latin monstrare "to show," from monstrum "omen, sign" (see monster). Meaning "to collect, assemble" is early 15c.; figurative use (of qualities, etc.) is from 1580s. To muster out "gather to be discharged from military service" is 1834, American English. To muster up in the figurative and transferred sense of "gather, summon, marshal" is from 1620s. Related: Mustered; mustering.
late 14c., "action of showing, manifestation," from Old French mostre "illustration, proof; examination, inspection" (13c., Modern French montre), literally "that which is shown," from mostrer (see muster (v.)). Meaning "act of gathering troops" is from c.1400. To pass musters (1570s) originally meant "to undergo military review without censure."