Why is the ninth month called September?
1630s, from Latin quiescentia, from quiescere (see quiescent).
c.1600, from Latin quiescentem (nominative quiescens), present participle of quiescere, inchoative verb formed from quies "rest, quiet" (see quiet (n.)).
quiescent qui·es·cent (kwē-ěs'ənt, kwī-)adj. Being quiet, still, or at rest; inactive.