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Tomorrow, for the first time this century, MPs will sit in the House of Commons on a Saturday.
They will fill Westminster’s green benches to decide on whether Boris Johnson’s last-minute Brexit deserves a historic seal of approval.
But the Prime Minister, whose Conservative Party lacks a majority among the House’s 650 seats will face a deeply divided Parliament.
His hopes of pushing through his “great new Brexit deal” rest on the backing of competing Tories and opposition members after the Government’s DUP allies withdrew their support.
So what is the plan for tomorrow? And what are the potential outcomes?
When will the vote take place?
The precise time has not yet been confirmed, but Parliament will sit from 9.30am on Saturday, October 19.
Mr Johnson will address the House in a statement, before the main debate begins. It was originally expected to finish at 2.30pm but it could go on into the evening.
MPs will cast their votes some time in the late afternoon or evening.
What exactly are they voting on?
Essentially, the main motion is to approve or reject the deal he reached with the EU. Mr Johnson is framing Saturday’s vote as a choice between his deal or a no-deal Brexit or no Brexit at all.
However, the choice is inevitably more complex than that.
Oliver Letwin has put down an amendment that could change people’s calculations. It states that the House “withholds approval” of the deal until such time that the legislation to enact the deal has been passed.
The reason for this is that a straightforward approval would prevent the Benn Act (which obliges the PM to request an Article 50 extension if he does not have an approved deal) from kicking in.
The Letwin amendment means Mr Johnson is forced to apply for an extension regardless of the result.
What are the potential outcomes?
MPs approve the Letwin Amendment
This is voted on first and has the backing of Labour and the Tory rebels, so it could pass. Unless Mr Johnson finds a way to ignore it, he may be in the embarrassing position of having to ask for a Brexit delay beyond October 31.
MPs approve the deal
If the gets his deal over the line, Mr Johnson can proceed with his plan to leave the EU at 11pm on October 31.
But first, legislation must be passed at Westminster and the agreement must be ratified by the European Parliament.
The latter could happen as early as next week when EU lawmakers meet in Strasbourg, but Brexit coordinator Guy Verhofstadt insists they will take as much time as necessary to examine the divorce deal in sufficient detail, even if this spills past the October 31 deadline.
The European Parliament will only begin its work once Westminster has passed the deal.
MPs reject the deal
If his deal is not approved, the PM is obliged by law to request an extension to the Brexit deadline, crushing his promise to deliver by October 31 “do or die”.
Under the so-called Benn Act, Mr Johnson will have to write to EU leaders asking for a three-month extension to Article 50, meaning the UK will not leave the bloc until January 31 at the earliest.
There has been some concern the Government has been searching for a loophole to avoid having to request the extension.
For example, it’s been suggested that Mr Johnson could request the delay in one letter, but tell the EU to ignore his request in another.
If the PM refuses point-blank to request the extension he could face a legal challenge.
What happens if the EU rejects the extension?
If the EU refuses to extend Article 50, the UK will crash out of the bloc on October 31 at 11pm without a deal.
This means the UK will immediately exit the customs union and single market and default to World Trade regulations.
Many politicians and businesses say this would cause terrible damage to the economy, but others accuse them of scare-mongering.
How many votes does the PM need to secure his deal?
He needs 320 votes out of 650.
So what are his chances?
To secure the majority he needs to secure his deal, the PM needs votes from several groups – 28 hardline Conservative Eurosceptics who belong to the so-called Spartan wing of the European Research Group; 21 former Tory rebels who lost the whip but haven’t yet defected; 19 sitting Labour MPs who have previously suggested they might be willing to back a deal, and a handful of former Tory and Labour MPs sitting as independents.
His 10 DUP allies have already confirmed they will not offer their support, as have the Labour Party, SNP and the Liberal Democrats.
Some 287 Conservative MPs are eligible to vote but some will not back the deal. Around 17 or 18 whipless Tories will back the deal, along with independent former Tories such as Nike Boles. Only a few Labour MPs have so far signalled they will vote for it, including Ronnie Campbell, Jim Fitzpatrick and John Mann. Others are pondering whether to do so.
Mr Johnson is currently at least a dozen to 15 votes short of victory.
Today, he is fighting a race against time to secure their backing.
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