It depends on your standards. Astronomically you can either start counting Summer at the Solstice (Autumn/Fall, Winter and Spring at the respective Equinoces/counterpart Solstice, the common practice) or indeed align that date (or datetime) with the centre or use the midway eighth-marks betwixt the quarter-points (a bit rarer, and differs according to how you calculate it due to annual orbital fluctuations and the rest).
Meteorologists tend to subscribe to something straight and simple like "June July August" (92 days) as Summer, and likewise ascribe three whole months in their entirety to each of the other seasons (91 days, 90/91 days, 92 days - making verdant spring and heady summer longer than dreary autumn or bleak winter - but the other way round in the southern hemisphere, of course).
Children are often tied to the (local) alignment of elements in the Academic Year, with the long, warm Halcyon Days Of Summer (whatever the weather actually serves) starting the instant you're set free from the last class of that final term/semester of the year (though itself maybe called the "summer term", somehow "winter term" is the one that doesn't exist in my own memory, the year roughly thirded amongst the others, with Easter Break somehow accomodated despite its fidgetting around the calendar) , Autumn possibly when they return , etc. And even several decades on, from the dozen or so years only of such dictated alignment, I can still feel some of that quite specific ebb-and-flow.
Practically, your locale might celebrate spring as the sudden cracking of the river ice so that it starts flowing again, or the ability to use the harbour again for early-season fishing in the (ant)arctic waters. That's probably even more prone to wether-blips and climatic changes than the places that track by the blossoming of a certain tree, and other natural events (first frost?) for other season changes. Or assumed lengths keyed by the cues/hindsight of when that cue (the last frost?) clearly occured.
"Mid-Summer's Solstice" is clearly more from a non-celestial Summer, to which the Solstice concerned is (a)mid(st), within, an occurance of note but not in itself a definition... A mash-up with the term for that special day within the period of Summer that has traditionally been marked with whatever rituals, festivals or feasts as have been set in stone (or hay) as Mid-Summer's Day. Ditto with Mid-Winter, yet I think the Equinoces are never considered Mid- of anything. The maypoles and corn-dollies and daffodills and pumpkins tend to just get smudged around to a more convenient point innthe calendar than necessarily guided by the solar passage across the ecliptic, etc.
Still, I do like pointing out that days only get shorter in (my prefered version of) Summer. Or I may out-pedant someone by saying that "no, they're all 24 hours long". Or I may further out-pedant someone who so tried to out-pedant me, that way, by pointing out that the Moon is lengthening days (in 50 millenia, or so, Leap Seconds would not just be required every few end-of-years, or even at the end of every month, but Every. Single. Day...), those of us not already more tied to Mars's clock-cycle (already notably longer) or some other dissassociated (or delinked) clock.
And let us not forget the equitorials, and those not far from, for whom the key marker and season is marked by the likes of something like monsoons or their absence.