January 26 is the the date in 1788 on which Captain Arthur Phillip raised the flag of Great Britain in Port Jackson, later known as Sydney Cove and now known as Circular Quay on the south shore of Sydney Harbour between the bridge and the opera house.
It was the date on which the First Fleet arrived with its human cargo, including convicts, plus stores and material, to start building what would become European Australia.
It is not the date on which Captain Cook arrived. He "claimed" the land for Britain on August 22, 1770.
In the early years of the new colony, it came to be called Foundation Day. After all, Australia as a nation didn't formally begin until federation on New Year's Day in 1901.
As the century developed, the idea of Australia Day took hold and by 1935 it had been adopted by all the states and territories. It didn't become a public holiday, though, until 1994.
The day evolved from a celebration by emancipated convicts into what is meant to be a celebration of Australia and the good fortune of Australians to be Australian.
Why is it a big deal?
It's a day off, of course, but it's also full of symbolism, some of it contentious.
The official Australia Day organisation says: "On Australia Day we celebrate all the things we love about Australia: land, sense of fair go, lifestyle, democracy, the freedoms we enjoy but particularly our people.
"Australia Day is about acknowledging and celebrating the contribution that every Australian makes to our contemporary and dynamic nation.
"From our Aboriginal and Torres Strait Islander people - who have been here for more than 65,000 years - to those who have lived here for generations, to those who have come from all corners of the globe to call our country home."
What could be the problem with that?
Some indigenous people argue that it is a celebration of the day on which the destruction of their culture began.
As the actor and writer Nakkiah Lui wrote: "We do not celebrate the coming of the tall ships in Sydney's harbour.
"Instead, we mourn the declaration of Australia as terra nullius (land that belongs to no one) as well as those who have died in massacres, those who were dispossessed of their land and homes, those were denied their humanity, those who were shackled, beaten, sent to prison camps and made to live in reserves.
"We mourn those who have died in the resistance."
Other Indigenous people are not so forthright in their criticism but still do not feel that the day embraces them despite the rhetoric about "celebrating the contribution that every Australian makes to our contemporary and dynamic nation".
What do Australians think?
As you might imagine, opinion is split, though most people polled four years ago felt positively about Australia Day as it is - but not a majority among Aboriginal Australians.
Two-thirds of the total of those polled (68 per cent) felt positive about it, with 26 per cent either "indifferent" or having "mixed feelings".
But among Aboriginal Australians and people from the Torres Strait Islands, less than a quarter (23 per cent) felt positive and 61 per cent felt negative or had mixed feelings.
Why not change the name or date?
The name and the date are politicised and controversial, with entrenched attitudes.
Those opposed to change argue that it would devalue the contribution of European culture and technology to this continent, for the benefit of all its peoples, albeit with downsides.
And symbols are powerful things. Witness the controversy last week when Scott Morrison said that that initial January 26 "wasn't a particularly flash day" for British convicts when they landed.
He was opining that Cricket Australia should "stick to cricket" after a recommendation to drop the words "Australia Day" from promotions for Big Bash League fixtures on January 26.
By the way, January 26 was not the day when the First Fleet landed in Australia. It actually landed in Botany Bay around the corner a few days earlier and then found an even better harbour.
But that's a technicality. It's the symbolism of settlement by white people which some Aboriginal people find offensive. Some call it "Invasion Day", "Day of Mourning", "Survival Day'" or "Aboriginal Sovereignty Day".
None of those seem likely to be generally accepted as alternatives but other less loaded versions on a different day might work (perhaps Wattle Day on September 1).
The first Australia Day was actually on July 30.
It's been alleged that having Australia Day on a Tuesday, Wednesday or Thursday means absenteeism from work on a large scale, and a permanent Monday or Friday may be economically better.
Fremantle has switched the celebrations to a few days later. "One Day in Fremantle is presented by the City of Fremantle and is a free, family-friendly event that provides a culturally-inclusive alternative to traditional Australia Day celebrations," the city authorities say.
It's a party
From the early days of settlement, the day has been a celebration.
In 1817, The Sydney Gazette and New South Wales Advertiser reported : "On Monday the 27th a dinner party met at the house of Mr. Isaac Nichols, for the purpose of celebrating the Anniversary of the Institution of this Colony."
"At 5 in the afternoon dinner was on the table, and a more agreeable entertainment could not have been anticipated. After dinner a number of loyal toasts were drank, and a number of festive songs given; and about 10 the company parted, well gratified with the pleasures that the meeting had afforded."
We don't know how much drink they consumed.
Two hundred years later, Diana Egerton-Warburton of the Australasian College for Emergency Medicine said that doctors regarded Australia Day as ''the peak day for the national sport of getting drunk and injuring yourself and other people''.