So, I guess in Australia they have winter in the hottest part of the year.
If you can’t tell them apart, they’re not really separate seasons then.
“Season” is a word that means different things in different contexts and populations.
Some people look at seasons as “quarters of the year”.
Winter = Dec - Feb
Spring = Mar - May
Summer = Jun - Aug
Fall = Sep - Nov
Other definitions of seasons use the soltices and equinoxes (equini?).
Yet another considers roughly by temperature. Winter kicks in at the first snow. Spring when it gets warm enough to start melting. Summer when it gets “so hot”. Fall when the leaves start to turn.
Or probably a number of other patterns that are significant enough to someone to say “aha, something is different, that’s how I know the season has changed”.
Bottom line: there’s no one definition* for the seasons.
*I was going to say “definitive definition” but decided that was repetitively redundant. 
Being a denizen of the Southeastern part of the United States, I firmly believe that autumn begins when the first college football games kick off, September 3rd for my Razorbacks. Go Hogs, Beat Bama!
It’s as good a definition as any other.
As a kid, the official first day of fall is the day you start school. And the last day of spring is the last day of school.
Astronomically speaking, the Earth has seasons. Whether you like it or not, the Earth is tilted on its axis of rotation, and its path around the sun is not a perfect circle.
The affects everyone on the planet, in Cameroon or the Arctic, whether the most obvious form of weather makes it known or not. Even in the tropics, things are happening because of seasonal changes.
In the North East USA, we know the general seasonal differences for the area. In the tropics, it might reveal itself as a monsoon, or winds, or be much less obvious, but it is happening nonetheless.
The earth has seasons.
Only for that particular definition of season. It’s not applicable to every single circumstance.
Yes, there are seasons, even in the tropics. But it is nonsensical to call those seasons, “Fall”, “Winter”, “Spring” and “Summer”. You can call them “Rainy Season” and “Dry Season”, and yes, those are seasons. But “Rainy Season” isn’t “Winter” just because it’s winter in Alaska.
Here in upstate New York, Winter lasts from late September through mid-April.
And let’s not get started on how long hockey season lasts. Or whether it’s rabbit season or duck season.
In North America, seasons are determined by the calendar. I understand that in other parts of the world they are more determined by the weather you are likely to have. So in England, I think spring is considered to start in April or something like that. And autumn (more commonly called fall in North America, although “autumn” is used, it is considered more poetic) probably by early September.
As to why the different dates, well the length of the year (say from one autumnal equinox to the next) is a bit short of 356 1/4 days and it is that nearly a quarter day that causes the date to vary. Next year being a leap year, it should revert to Sept. 22. But it also advances during the century until the leap year is omitted on 2100 (assuming humanity survives till then). This is the result of that “bit short”.
Preach it, brother! I would add only that in a country the size of the US, there is no single day in which autumn can be said to arrive nationwide. Autumn starts a lot later in Memphis than in the uncivilized city of New York, and winter not only starts later in Memphis but is much briefer: January and February, basically. And of course, in other locales its silly to use the terms for temperate seasons.
It might also be noted that, even within a single year, the equinox will fall on different dates depending on where you live. This year’s equinox, at 09:04 UT, will fall on the 23rd for the majority of the planet, but in Hawaii and a few other Pacific islands it will still be the 22nd.
Says who? There’s no such thing as an “official” season anyway, as Cecil said.
Well, since the SDMB has continually drfited into wise-ass territory, and not actually thinking like they are in GQ, let’s all go back to the original post (OP). The OP is CLEARLY looking for a finer understanding that requires one to address the issue from an astronomical perspective.
The rest of the crap posted is then somehow used to remind people who are addressing the OP that you can expect warm breezes on December 15th somewhere near the equator and that they have no winter.
“Equinoxes” is perfectly acceptable, since we’re speaking English, but if you want to go all Latin on it, it’s “equinoces”. Oh, and you mispelled [appeasing Gaudere] “solstices”.
And there is some justification for using the solstices and equinoces to define the seasons. If you defined seasons according to, say, when certain constellations appear, or according to dates in the Julian calendar, then after a while, the hot times and cold times will drift out of sync with your defined seasons. This will even happen with Gregorian calendar dates, if you wait long enough. But a definition based on the solstices and equinoces won’t drift this way: Even if it’s a bit off, it’ll never get any more off.
Mind you, this still doesn’t completely resolve the question. For instance, Americans generally regard the summer solstice as the start of summer, but Brits generally regard it as the middle of summer. Both are equally based on the solstice.
OTOH, in some places, it’s ALWAYS nutria season.
Salt, pepper, sugar, chocolate sprinkles…
Which was pretty thoroughly answered in the first reply, with some follow up for the impact of Leap Years a few posts later. I think the discussion of various other interpretations of seasons is interesting and not too much of a hijack (although I acknowledge, my own wise-ass reply didn’t contribute anything useful).
Astronomers observe four seasons based on the revolution of the earth around the Sun. But many parts of the world, particularly the tropics, observe two seasons based on weather patterns.
In other parts of the world (as has been stated here and incorrectly dismissed) meteorologist demarc seasons by temperature. And farmers and ecologists would observe six seasons, based on growing cycles.
And of course, we don’t have summer in North America at the same time as they do in South America.
By that definition of “meteorological seasons”, New Orleans has only had one winter in the past 15 years (5 consecutive days below freezing) and some years, we don’t even have an autumn or spring (5 consecutive days below 10 degrees C or about 50 degrees F).