"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.