Why is the ninth month called September?
1570s, from Middle French irruption or directly from Latin irruptionem (nominative irruptio) "a breaking in, bursting in, invasion," noun of action from past participle stem of irrumpere, from assimilated form of in- "into, in, on, upon" (see in- (2)) + rumpere (see rupture (n.)). Frequently confused with eruption.
irruption ir·rup·tion (ĭ-rŭp'shən)
The act or process of breaking through to a surface.