More than three years since the United Kingdom voted 52-48 to leave the European project, Johnson was hoping to win parliament's approval for the divorce treaty he struck in Brussels on Thursday.
But his opponents have laid a parliamentary booby trap that could frustrate his plans, forcing him to send members of parliament home without voting on his deal on Saturday.
Former Conservative lawmaker Oliver Letwin, expelled from the party by Johnson, has proposed that the decision on whether to back a deal be deferred until all the legislation needed to implement it has been passed through parliament.
This proposal, which has cross-party support, will be put to a vote at the end of Saturday's debate. If the amendment is approved by parliament, Johnson's deal would not then be put to a vote on Saturday.
The legislation would be brought back again on Monday, according to the BBC's political editor. That would be a major setback that would leave Brexit, yet again, up in the air.
The Letwin move, aimed at preventing the United Kingdom from somehow dropping out of the EU without a deal, would in effect force Johnson to delay Brexit until all the laws needed to leave have passed through parliament's multi-stage approval process.
Even though Johnson believes this can be achieved 31 by October, others think it would need a short 'technical' delay.
A law passed by Johnson's opponents obliges him to ask the EU for a Brexit delay until Jan. 31, 2020 unless he has secured approval for his deal by the end of Saturday.
"My aim is to ensure that Boris’s deal succeeds," said Letwin, expelled from Conservative Party for refusing to back Johnson's plans to leave the EU with or without a deal.
But Letwin wanted "an insurance policy which prevents the UK from crashing out on 31 October by mistake if something goes wrong during the passage of the implementing legislation".
In a day of Brexit high drama, lawmakers convened for the first Saturday sitting since the 1982 Argentine invasion of the Falklands, while hundreds of thousands of people are due to march to parliament demanding another referendum.
Johnson depicted the vote as the last chance to secure an orderly Brexit. Though he is obliged by law to seek a Brexit delay if his deal falls, Johnson said the United Kingdom would still leave on Oct. 31. He didn't explain how.
"There have been any number of false dawns. Deadlines for our departure have come and gone," Johnson, the face of the 2016 campaign to leave the EU, wrote in Britain's best-selling newspaper, The Sun. "Today can be the day we get Brexit done."
The so-called Brexit "Super Saturday" tops a frenetic week which saw Johnson confound his opponents by clinching a new Brexit deal although his Northern Irish allies opposed it.
In a divided parliament where he has no majority, Johnson must now win the support of 320 lawmakers to pass his deal.
If he wins the vote, Johnson will go down in history as the leader who delivered a Brexit - for good or bad - that pulls the United Kingdom far out of the EU's orbit.
Should he fail, Johnson will face the humiliation of Brexit unravelling after repeatedly promising that he would get it done - "do or die" - by Oct. 31.
Johnson was making a statement to lawmakers on Saturday, after which there will be a debate and then votes on amendments and finally - if all goes according to the government's plan - his deal.
'Get Brexit done'
Johnson won the top job by staking his career on getting Brexit done by the latest deadline of Oct. 31 after his predecessor, Theresa May, was forced to delay the departure date. Parliament rejected her deal three times, by margins of between 58 and 230 votes earlier this year.
He said Saturday's vote was the last chance to get Brexit done with lawmakers facing the option of either approving the deal or propelling the United Kingdom to a disorderly no-deal exit that could divide the West, hurt global growth and trigger violence in Northern Ireland.
"Today we MPs (Members of Parliament) have the chance to free you from the never-ending Brexit saga and move this country forward," Johnson wrote. "In less than two weeks, on October 31, we would be out of the EU."
To win the vote, though, Johnson must persuade enough Brexit-supporting rebels in both his own Conservative Party and the opposition Labour Party to back his deal. His Northern Irish allies and the three main opposition parties oppose it.
In a boost for Johnson, some influential hardline Brexit supporters such as Mark Francois and Iain Duncan Smith said they would support the deal.
Steve Baker, the head of a hardline Brexit faction in the Conservative Party, has told his European Research Group allies that they should vote for Johnson's deal.