Why is the ninth month called September?
1580s, a method of 10-year banishment in ancient Athens, by which the citizens gathered and each wrote on a potsherd or tile the name of a man they deemed dangerous to the liberties of the people, and a man whose name turned up often enough was sent away. From Middle French ostracisme (16c.), Modern Latin ostracismus, or directly from Greek ostrakismos, from ostrakizein "to ostracize," from ostrakon "tile, potsherd," from PIE *ost-r-, from root *ost- "bone" (see osseous). The Greek word is related to osteon "bone," ostreion "oyster" (and cognate with German Estrich "pavement," which is from Medieval Latin astracus "pavement," ultimately from Greek ostrakon).
A similar practice in ancient Syracuse (with banishment for five years) was by writing names on olive leaves, and thus was called petalismos.
political practice in ancient Athens whereby a prominent citizen who threatened the stability of the state could be banished without bringing any charge against him. (A similar device existed at various times in Argos, Miletus, Syracuse, and Megara.) At a fixed meeting in midwinter, the people decided, without debate, whether they would hold a vote on ostracism (ostrakophoria) some weeks later. Any citizen entitled to vote in the assembly could write another citizen's name down, and, when a sufficiently large number wrote the same name, the ostracized man had to leave Attica within 10 days and stay away for 10 years. He remained owner of his property. Ostracism must be carefully distinguished from exile in the Roman sense, which involved loss of property and status and was for an indefinite period (generally for life).