"hot season of the year," Old English sumor "summer," from Proto-Germanic *sumra- (source also of Old Saxon, Old Norse, Old High German sumar, Old Frisian sumur, Middle Dutch somer, Dutch zomer, German Sommer), from PIE root *sm- "summer" (source also of Sanskrit sama "season, half-year," Avestan hama "in summer," Armenian amarn "summer," Old Irish sam, Old Welsh ham, Welsh haf "summer").
As an adjective from c. 1300. Summer camp as an institution for youth is attested from 1886; summer resort is from 1823; summer school first recorded 1810; theatrical summer stock is attested from 1941 (see stock (n.2)). Old Norse sumarsdag, first day of summer, was the Thursday that fell between April 9 and 15.
"ancient Celtic festival celebrated on the first of November," 1888, from Irish samhain (Gaelic samhuinn), from Old Irish samain, literally "summer's end," from Old Irish sam "summer" (see summer (n.1)) + fuin "end." It marked the start of winter and of the new year.
It is the American version of British All-Hallows summer, French été de la Saint-Martin (feast day Nov. 11), etc. Also colloquial was St. Luke's summer (or little summer), period of warm weather occurring about St. Luke's day (Oct. 18). An older and simpler name for it was autumn-spring (1630s).
"the middle of summer, the period of the summer solstice," Old English midsumor, from mid (adj.) + sumor "summer" (see summer (n.1)). Midsummer Day, an English quarter-day, was June 24 (Feast of the Nativity of St. John the Baptist). Astronomically, midsummer falls on June 21, but the event traditionally was reckoned in northern Europe on the night of June 23-24 (with summer beginning at the start of May). Midsummer night was an occasion of superstitious practices and wild festivities.