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rep + good player

Okay that's an exhaustive but super-simplified explanation of why the "start of summer" changes and why the phrase, "What day does summer start this year?," is a thing. The solstice is around the 20-22 of June in the Northern Hemisphere (depending on time zone and what year it is) because of the Gregorian calendar being quite accurate but not 100%, and because "seasons" are a thing that exist not only on Earth but on other celestial bodies. Anyway, hope that helped! There's a LOT more to it and a lot of interesting stuff because it involves thousands of years of science, and some AMAZINGLY brilliant people throughout history in many different cultures, but yeah that's the gist of it.

Sure, it's actually not too hard to explain, and although the measurements require a lot of knowledge of math, the explanation doesn't. The summer solstice is the day of the year where that specific part of the earth (based on the tilt and rotation) is at the maximum amount, meaning it receives the most sunlight.

This applies not only to earth but to every other celestial body, which is how we also determine "seasons" on other planets (such as seasons on Neptune lasting like 40 earth years!), and can even apply to satellites like our own moon which experience one day as being almost 30 of our own (due to it's waaaay more shallow tilt). So since 3 "months" wouldn't mean anything to any other planet or celestial body of any time anywhere else other than Earth (and it BARELY matters here, to be honest, which I'll get into later), the astronomical measurement of seasons has been the gold standard for literally a few thousand years (including even the Mayan calendar, which was divided into 18 months but still used the solstices to keep track of major seasons and cultural/religious markers).