Some 60,000 Orangemen from across Northern Ireland will rally at eight major centres for the 313th anniversary of the Battle of the Boyne today.
They are expected to be joined by up to 40,000 visiting Orange Order brethren from Scotland, England and further afield.
An Imperial Council of the order is to be convened in Glasgow later this month and Orangemen from New Zealand, the US, Canada, Ghana, Togo and even Poland are expected.
Some 800 bands will lead members of around 90 Orange districts on the various marches. The main lodges in the Republic have already held their parade earlier this month in Rossnowlagh, Co Donegal.
The main Belfast march will leave Carlisle Circus in the north of the city this morning and parade through the centre to the field at Edenderry between Belfast and Lisburn.
Orangemen there will be addressed by Mr Fraser Agnew, a former UUP member of the Assembly and a previous District Master of the Orange Order.
The Grand Master, Mr Robert Saulters, who usually speaks at the Belfast parade, will speak in Ballymena, Co Antrim, where a significant turnout is expected. The main Co Antrim march will be in Glengormley, the first rally there in 10 years.
Mr Dennis Watson of the Portadown district will speak at the main Co Armagh demonstration in Markethill. The Portadown Orange lodge at the centre of last weekend's Drumcree banning controversy will march here.
The Rev Martin Smyth, president of the Ulster Unionist Party, and a senior Orange figure, will address the meeting in Crossgar, Co Down. Fellow Ulster Unionist and anti-Belfast Agreement colleague, Mr Willie Ross, will speak at the Derry parade in Limavady.
The main lodges in Derry city will rally here following an agreement with nationalist residents to rotate the venues for the city's Orangemen. Derry will not host the Twelfth demonstration again until 2005. Another large Orange demonstration in Co Derry will converge on Garvagh.
There will also be sizeable demonstrations in Fintona, Co Tyrone; and in Lisnaskea, Co Fermanagh.
The Twelfth marks the high point of the marching season and is usually accompanied by heightened tensions between loyalist and nationalist communities in interface areas.
This year has so far witnessed less violence and previous controversial marches have largely passed off peacefully.
In Belfast, the so-called Tour of the North parade, which earlier this month followed a route past key flashpoints in the north of the city, was virtually trouble-free.
Another key march in the Springfield Road area of west Belfast also passed off peacefully. This set the scene for a quiet Drumcree parade last Sunday.
The Lord Mayor of Belfast, Mr Martin Morgan, yesterday appealed for calm over the weekend. The SDLP north Belfast councillor called on all those involved in parading over the coming days to exercise "level-headedness and common sense".
He said the next few days were "notoriously difficult ones". "At this time of year tensions are normally high and it is important that all those with influence in the community exercise that influence to ensure that parades pass off without incident," he added.
Political representatives and community figures are understood to have been working hard to keep a lid on community tensions as the marching season reaches a climax.
Despite some sporadic violence in east Belfast on three consecutive nights earlier this week, this summer has seen a marked decrease in street violence compared to last year.
The Parades Commission is to allow a "feeder parade" by the Orange Order to pass through the Ardoyne area of north Belfast this morning.
Another feeder parade banned from the lower stretch of the Ormeau Road in south Belfast is expected to hand over a letter of protest at Ormeau Bridge to police officers.
A spokesman for the Grand Orange Lodge of Ireland said some demonstrations were organised with families in mind and would strive to create a more carnival atmosphere than normal.