Why is the ninth month called September?
"hot season of the year," Old English sumor, from Proto-Germanic *sumur- (cf. Old Saxon, Old Norse, Old High German sumar, Old Frisian sumur, Middle Dutch somer, Dutch zomer, German Sommer), from PIE root *sem- (cf. Sanskrit sama "season, half-year," Avestan hama "in summer," Armenian amarn "summer," Old Irish sam, Old Welsh ham, Welsh haf "summer"). Old Norse sumarsdag, first day of summer, was the Thursday that fell between April 9 and 15.
Summer camp is attested from 1893; summer resort is from 1832; summer school first recorded 1860; theatrical summer stock is attested from 1942.
"horizontal bearing beam," late 13c., from Anglo-French sumer, Old French somer "main beam," originally "pack horse," from Vulgar Latin *saumarius, from Late Latin sagmarius "pack horse," from sagma "packsaddle" (see sumpter).
"to pass the summer," mid-15c., from summer (n.1). Related: Summered; summering.
warmest season of the year, between spring and autumn. In the Northern Hemisphere, it is usually defined as the period between the summer solstice (year's longest day), June 21 or 22, and the autumnal equinox (day and night equal in length), September 22 or 23; and in the Southern Hemisphere, as the period between December 22 or 23 and March 20 or 21. The temperature contrast between summer and the other seasons exists only in middle and high latitudes; temperatures in the equatorial regions generally vary little from month to month. For physical causes of the seasons, see season.