Video report by ITV Correspondent Neil Connery
Abortion and same-sex marriage will become legal in Northern Ireland at midnight.
The Stormont Assembly will sit for the first time since the power-sharing executive collapsed in January 2017 amid a bitter dispute between Sinn Fein and the DUP.
In a bid to protest the reforms, some MLAs will return to Stormont on Monday, but unless a power-sharing executive can be formed before midnight, same-sex marriages will be able to take place from January, while women will have greater access to terminations from April.
The afternoon sitting will be largely symbolic, as the Assembly is unable to perform its legislative functions without a ministerial executive in place.
MPs at Westminster successfully amended a Government bill in the summer to include measures to end the near blanket prohibition on abortion and introduce same-sex marriage in Northern Ireland.
Same-sex marriage and abortion will become legal, unless the devolved executive is revived prior to the midnight deadline - a turn of events that is extremely unlikely.
Rival MLAs have branded it a meaningless stunt, the members who signed the 30-strong recall petition have insisted it will provide a forum to voice opposition to the imminent decriminalisation of the key legislation.
While DUP and UUP members are set to attend, the rest of the chamber's benches will be largely empty.
Sinn Fein has made clear it will not turn up to a sitting it has described as a circus.
Anti-abortion and pro-choice campaigners gathered at the front of Parliament Buildings on Monday morning to voice their contrasting views on the emotive issue.
Sarah Ewart, who has become a vocal advocate for reform since having to travel to England for an abortion after receiving a diagnosis of fatal foetal abnormality, welcomed the law change.
"I can't even put into words how this is going to be for people, I have had people contacting me, as close as last week, who are going through this, who are thanking me, because they now know they are going to have a choice here," she told ITV News.
"It's such a scary thing to be going through, especially to be sent away, so to have this here at home, it's not going to make it easier, but it will make it a little less stressful."
The Executive Formation Act 2019 comes into effect at midnight on Monday, when laws on same-sex marriage and abortion change.
The 19th Century laws that criminalise abortion lapse and the Government will assume responsibility for introducing new regulations to provide greater access to terminations in the region by next April.
In the interim period, women will be offered free transport to access abortion services in England.
Under the Act, same sex marriage will become legal in Northern Ireland in January, with the first wedding expected the following month.
Grainne Teggart from Amnesty International said the law change would pave the way for a more "compassionate" system.
"From midnight tonight history will be made, these oppressive laws that have policed our bodies and our healthcare will be brought to an end," she said.
"Finally our rights and our healthcare are being brought into the 21st century."
But anti-abortion activists held up placards stating that the decriminalisation was not in their name.
Activist Clive Johnston, from Strabane, warned of the consequences of decriminalisation.
"In today's world the most dangerous place to be is actually in the womb of a woman," he said.
"The Government is culpable in actually taking part in what amounts to the killing of babies in the womb."
While Bernadette Smyth, of Precious Life campaign group, told ITV News: "How dare Westminster impose such horrific open warfare on unborn children, this is very very wrong for another Government to impose the opinions of that Government on another Government who are opposed to abortion."
The last time the Assembly sat was March 13, 2017 in the wake of a snap election caused by the implosion of the devolved institutions two months earlier amid a row over a botched green energy scheme.
The sitting was only for the 90 freshly-elected MLAs to sign the roll of membership and the Assembly has not reconvened since.