midsummer n.

"middle; being the middle part or midst; being between, intermediate," Old English mid, midd from Proto-Germanic *medja- (source also of Old Norse miðr, Old Saxon middi, Old Frisian midde, Middle Dutch mydde, Old High German mitti, German mitte, Gothic midjis "mid, middle"), from PIE root *medhyo- "middle."

By late Middle English probably felt as a prefix only, and now surviving in English only as a prefix (mid-air, midstream, etc.). Prefixed to months, seasons, etc. from late Old English. As a preposition, "in the middle of, amid" (c. 1400) it is from in midde or a shortened form of amid (compare midshipman) and sometimes is written 'mid.

"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.