Why is the ninth month called September?
1640s, "musical performance at night in open air" (especially one given by a lover under the window of his lady), from French sérénade (16c.), from Italian serenata "an evening song," literally "calm sky," from sereno "the open air," noun use of sereno "clear, calm," from Latin serenus "peaceful, calm, serene." Sense influenced by Italian sera "evening," from Latin sera, fem. of serus "late." Meaning "piece of music suitable for a serenade" is attested from 1728.
1660s, from serenade (n.). Related: Serenaded; serenading.
originally, a nocturnal song of courtship, and later, beginning in the late 18th century, a short suite of instrumental pieces, similar to the divertimento, cassation, and notturno. An example of the first type in art music is the serenade "Deh! vieni alla finestra" ("Oh, Come to the Window"), from Wolfgang Amadeus Mozart's Don Giovanni. The instrumental serenade gradually lost its association with courtship and became (about 1770) primarily a collection of light pieces such as dances and marches suitable for open-air, evening performance