The question posed does of course have a very short answer – the UK is leaving the EU on that date because a majority of voters in the Brexit referendum of June 2016 backed the Leave campaign.
Theresa May triggered Article 50 in March of last year before she had even conceived of her snap general election, never mind called it.
Given the shocking result that saw the Prime Minister lose her House of Commons majority, it is remarkable how little has in fact changed in the day to day business of politics.
Mrs May remains Prime Minister, Jeremy Corbyn remains Labour leader, and the country remains on course to leave the EU a year from today.
We look at why the seemingly inauspicious date of March 29 2019 was chosen, what problems it has caused, and how it could become an issue as the date looms larger.
Given that she is now unable to move certain ministers due to potential rebellion, it is easy to forget how powerful Mrs May appeared this time last year.
Polls showed that if a general election, then thought unlikely, was called, the Prime Minister was likely to massively increase her majority.
That is not to say, however, that the senior politician who had done the most to remain above the fray during the bitter referendum battle of 2016, was not under pressure.
Those pushing for a ‘hard Brexit’ don’t take much to show their agitation, as last week’s bizarre fish-dumping protest on the River Thames showed.
Before Article 50 was triggered, those arch-Brexiteers were keen for the Prime Minister to push the button on the official mechanism for leaving the EU as soon as possible.
Mrs May gave them a boost and shot down the hopes of continuity Remainers by triggering Article 50 and declaring that there was ‘no turning back’ on Brexit.
The right date?
The backlash to Mrs May’s notice to the EU that we were leaving the organisation was almost immediate.
From the off, voices within the European Union, both the organisation and in member states, were sceptical about how quickly, if at all, a satisfactory deal could be hammered out.
A couple of big ticket speeches from Theresa May, and several bi-lateral meetings later, and some of the biggest issues have still to be tackled.
Mrs May pulled the trigger against the advice of senior civil servants and diplomats, such as former representative to the EU Sir Ivan Rogers, who told a Commons Committee that the UK could be ‘screwed’ by triggering Article 50 before agreements were in place.
As battles on the Irish border and citizens’ rights continue to be kicked down the road, the quicker that departure date looms into focus, and the more that triggering Article 50 could look like a strategic mistake.
Is it set in stone?
The mounting issues with a year to go would suggest that some flexibility on the date of departure wouldn’t go amiss.
Just this month, a furious row has erupted over the fact that the UK will be subject to the common fisheries policy for at least two years after the country has left the EU.
It has also been somewhat forgotten that each of the member states has to individually ratify the final deal with the UK.
That is 27 parliaments, 27 governments, 27 different sets of circumstances that any deal with Theresa May’s government will have to navigate.
However, it is hard to understate the outcry if, for whatever reason, the UK delays their date of departure from the EU.
If Theresa May even hints at such a move, there will be a cabinet revolt, a furious media, and quite possibly a leadership challenge, within a matter of days, if not hours.
The fishing row shows the UK Government would rather remain part of EU institutions than delay Article 50.
For better or worse, the UK will leave the EU a year today, no matter the consequences.