Ten of thousands of Orangemen will parade in demonstrations across Northern Ireland for the annual Twelfth celebrations.
Around 50,000 Orangemen and band members are expected to attend 18 Twelfth demonstrations organised by the Grand Orange Lodge of Ireland.
On the biggest day in the Orange Order year the only flagship parade will be held in Londonderry. It is estimated around 10,000 spectators will line the route.
Parades in Belfast, Bangor, Newtownhamilton and Castlecaulfield will also draw tthe crowds.
Beginning at 10am, the flagship parade marks the culmination of Order events in Derry as part of the UK City of Culture.
Orange Order director of services Dr David Hume said: "The fact that this will be the only flagship of 2013 highlights the importance which the Grand Orange Lodge of Ireland places on the UK City of Culture status which has been accorded to Londonderry and Northern Ireland.
"The organisers of the Twelfth in Londonderry are offering a varied festival and their demonstration takes place in one of the most historic venues in Ulster."
Tourism Minister Arlene Foster said the year offers "unprecedented opportunites" for tourism in the north west. She said: "The flagship concept focuses on broadening the appeal of parades to attract a tourism audience. I know those involved in the demonstration will provide a varied and vibrant programme of events for visitors to enjoy."
Up to 20,000 spectators are expected in Castlecaulfield, Co Tyrone, to enjoy the festivities there.
Orangemen from seven districts will take part, with 70 lodges and 50 bands.
Tens of thousands of people are also expected to attend the Twelfth parade in Bangor, Co Down.
Glengormley will host the East Antrim Combine Twelfth.
Nine district lodges, 70 private lodges and 4,000 members will be led by 60 bands.
Twelfth celebrations in Belfast mark the 150th anniversary of the County Grand Orange Lodge of Belfast.
Leaving Carlisle Circus at 10am, Orangemen will make their way along Clifton Street to Belfast City Hall.
A wreath-laying ceremony will take place at the Cenotaph at the City Hall before the parade will make its way to the demonstration field in Barnett's Demesne.
Lodges from Scotland, England and the Republic's counties of Monaghan, Donegal, Cavan and Leitrim will join celebrations throughout Northern Ireland.
The Independent Orange Order will hold its demonstration at Portglenone in north Antrim.