Why is the ninth month called September?
early 15c., "fighting, engaged in warfare," from Middle French militant "fighting," from Latin militantem (nominative militans), present participle of militare "serve as a soldier" (see militate), originally especially in Church militant. Related: Militantly.
"one engaged in war or strife," c.1600, from militant (adj.); in a political sense, it is attested by 1907.