The answer to this question depends on what you mean by autumn.
(and which hemisphere you're in, but let's take that as read).
Autumn always used to start at the autumnequinox, a true astronomical definition based on orbits, planetary tilt and daylight totals.
The autumn equinox is the precise moment the Sun appears to cross the celestial equator, the point at which the northern hemisphere begins to tilt away from the Sun resulting in less direct sunlight and cooling temperatures. It's also the day the sun rises due east and sets due west, wherever you happen to be in the world.
But that's a difficult concept to get your head round, not least because the first day changesyear on year.
Possible dates of the autumn equinox (London, BST)
1800-1807: 23rd September or 24th September 1808-1839: 23rd September only 1840-1899: 22nd September or 23rd September 1900-1935: 23rd September or 24th September 1936-1975: 23rd September only 1976-2063: 22nd September or 23rd September 2064-2095: 22nd September only 2096: 21st September 2097-2199: 22nd September or 23rd September [cycle repeats every 400 years, approximately]
Generally it's always either 22nd September or 23rd September.
Very rarely, in years before leap years at the start of certain centuries, it's 24th September (most recently 1803, 1807, 1903, 1907, 1911, 1915, 1919, 1923, 1927, 1931 and 1935). Exceptionally rarely, in leap years just before an end of century non-leap year, it can be 21st September (next occurrence, 2096).
This year it's 23rd September, at 8.50am BST.
But it'll still be 22nd September in Hawaii. An autumn equinox on 21st September is a lot more likely in Hawaii, and an autumn equinox on 24th September is a lot more likely in New Zealand
So officially, no, it's not autumn until Monday.
The first day of autumn always falls later in the month than the first day of the other seasons - the 22nd or 23rd, rather than the 20th, 21st or 22nd. This is because our seasons are of different lengths, because the Earth's orbit around the sun is an ellipse not a circle. The Earth is furthest from the Sun in July. Summer is therefore our longest season, at 93 days, 15 hours and 31 minutes (timed between solstice and equinox), which nudges the autumn equinox later in the month. Meanwhile autumn is only 89 days, 20 hours and 4 minutes long, which nudges the winter solstice back towards the 21st of the month.
Meteorologists don't like seasons that start on different dates each year, because that makes comparing data harder. Instead they choose to use periods of three complete months, based on average monthly temperatures, with summer as the warmest (June/July/August) and winter as the coldest (December/January/February).
And meteorological autumn started on 1st September.
Meteorological seasons run about three weeks ahead of astronomical seasons, prioritising the first day of an arbitrary calendar month over the phenomenon of an equinox or solstice. It makes sense if you're a meteorologist.
It seems the wider world, particularly the media, has started to embrace meteorological autumn because it's a lot easier to understand.
In the first week of September the internet was awash with chirpy press releases praising autumn activities, and social media bluster invoking misty mornings, icy windscreens and crunching brown leaves underfoot. No matter that temperatures at the start of September were in the 20°s, and nature's leafy carpet won't be with us before October, the PR calendar decreed open season on autumnal imagery and off they fired. Nobody looked out of the window, they just read the calendar.
But is it right to be counting the first three weeks of September as autumn instead of summer?
That's a very interesting question, which boils down to asking whether summer should include the first three weeks of September or the first three weeks of June. The evenings are always lighter in June. The sea is always warmer in September. Daytime temperatures are often quite similar. Overnight temperatures are surprisingly similar too. It's not so much of a done deal as you might think.
Another way of judging the seasons is by observing natural phenomena... so-called phenological indicators.
These cover a range of ecological and biological signs such as the leaves falling off the trees and the migration of birds to warmer climates.
The last swallows disappear between the end of August and the start of October. Ash trees start to change colour, and holly berries appear, in September or October. Oak leaves fall between the end of September and the end of November. Horse chestnuts become bare trees in November. Generally speaking.
But watching nature is not the most reliable way of determining a definite start date for the autumn season.
The Woodland Trust use 2007 as their reference year, because it had the most average conditions in recent years. Natural events are greatly influenced by weather and climate, causing autumn to start earlier or later than the standard astronomical or meteorological definitions, and climate change will only accelerate the variations.
Hence the need for a more reliable start date, be that 1st September or the autumn equinox.
And 1st September is inexorably taking over, because it doesn't require much thought, and modern social media very much appreciates the simple. What we're seeing is the realignment of the start of autumn, the dumbing down of the autumn equinox, as society embraces a decision meteorologists only made to improve the reliability of their data.
n.b. The equinox isn't even the day on which day and night are the same length...
At the equinoxes, the geometric centre of the Sun is above the horizon for exactly 12 hours. But sunrise is defined as the moment the upper edge of the sun's disc becomes visible above the horizon, and sunset refers to the moment the Sun's upper edge disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, has to be added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.
It's Wednesday for those who live between 45° and 60° North - that's the whole of the UK. It's Thursday at 40°N, Friday at 30°N and Saturday at 20°N.
...because the sun takes a finite time to set.
Here's the maths. The Sun's diameter is approximately ½ degree out of 360°, which equates to 1/720th of a day, or two minutes, which is how roughly long the sun takes to set at the equator. The duration of sunset increases as you get closer to the poles, where sometimes it never sets.
At the autumn equinox the setting sun hits the horizon at its steepest possible angle, and the duration of sunset can be calculated according to the formula 128/cosφ seconds (where φ is the latitude). Here in London, this particular weekend, the sun takes 3 minutes 26 seconds to set. It takes 3m20s in Plymouth, 3m30s in Birmingham, 3m40s in the Lake District, 3m50s in Edinburgh and 4m in Inverness.
At the summer solstice the sun descends at a shallower angle, so London's sunset takes 4 minutes 33 seconds. The formula for this is 142/cos(1.14φ), near enough, so long as you stay away from the poles.
Maybe there is good reason why the astronomical equinox is being overtaken by the meteorological.
Or maybe we're just lazy.
Whatever, we're still in the good half of the year until Monday, so get out there and enjoy today's late summer/early autumn heatwave.