It will celebrate the 319th anniversary of the Battle of the Boyne, and hopefully the weather will smile down on the crowds thronging the city.
It will all begin at 9am when the Armagh District Officers hold their Act of Remembrance and then go on the march to welcome the county's other 10 districts to the city that is, in effect, the focal point of the County of the Diamond.
Orangeism was founded in County Armagh - just a few miles from the city in the village of Loughgall in 1795 - and that has always given the county demonstrations here that special meaning.
Armagh District Master Roy Pillow said, "The last time the county's Twelfth graced Armagh was in 1997 and we're delighted to be the hosts once again.
"There are those who claim County Armagh's is the biggest Twelfth demonstration in Northern Ireland, indeed in the world - bigger even than Belfast - but I'm not getting into that argument. We're simply going to enjoy the day.
"We welcome all the Brethren, the bands, the drummers are all supporters, especially the overseas visitors and we want to make it an enjoyable day all round."
The visiting districts will be disembark from their buses throughout the morning at the Deanery, Portadown Road - near the Planetarium, march towards the Courthouse, and Armagh District will lead the way to the field at Hamiltonsbawn Road. The districts are Newtownhamilton (10 lodges), Killylea (12), Portadown (30), Lurgan (21), Richhill (5), Loughgall (22), Keady (7), Tandragee (21), Bessbrook (4) and Markethill (12).
The outward route is via College Street, English Street, Scotch Street, Barrack Street, Victoria Street, Drumadd Terrace and Kiln Hill.
And on the way back (from about 2.20 with Portadown first to leave the field) the 161 lodges will march via Barrack Hill, turn at the old Savings Bank (where they will be reviewed by the County Grand Officers), past Armagh Orange Hall on the Mall and back to the Portadown Road for the buses back home.
Of course, there will be the usual colour at the field, with refreshments, fun for the children, the meeting with old friends and the Act of Worship and Resolutions from the platform.
Mr Pillow is proud that the big day is part of several days of celebrations in Armagh. There was a Songs of Praise last week at St Mark's Church of Ireland on The Mall where the collection was given to the Southern Area Hospice in Newry.
There was a special tattoo on Tuesday night, with the emphasis on bands and Lambeg drums.
And tomorrow (Saturday) night - at the Twelfth field - there will be a special family night, with a laser shoot, a penalty kick competition, a bouncy castle, Scottish dancing, the 'Reality' Gospel Rock Band, a beacon will be lit at 10pm and it will climax with a fireworks display.
Again there will be a collection for the hospice, and that will complement the Grand Master's Charity with the money from that going to Kidney Research.
"I'd like to thank everyone who has helped organise the special events and the major operation for the Twelfth," said Mr Pillow. "We have a vibrant membership in Armagh and they work exceptionally hard. We couldn't do it without them.
"All that remains is to welcome all visitors to our city, and may they enjoy this special day in the Orange calendar."