A candle by the window will mark the outbreak of the First World War as troops across Northern Ireland gather for a vigil on Monday night.
August 4 is the centenary of Britain's declaration of armed conflict with Germany.
At the headquarters of 38 (Irish) Brigade in Lisburn members of the armed forces and their families will attend a service in which the lights will go out one by one until only a candle remains to light the stained glass window, a Ministry of Defence spokesman said.
Similar events are being held at other Army and RAF bases in Northern Ireland.
Another service will be led by Dean of Belfast John Mann, who will be joined at St Anne's Cathedral by other faith leaders.
A candlelit vigil and act of remembrance will be held later that night at the Cenotaph at Belfast City Hall.
First Minister Peter Robinson will be present at the commemorative service along with a senior member of the Irish Government and other community leaders from across Ireland.
Representatives of the Royal British Legion and regimental associations of the army will also attend.
The candlelit vigil will coincide with a similar event at Westminster Abbey and in other regional capitals across the UK. It will be open to the public and those planning to attend are encouraged to bring a candle.
The ceremony will include a short act of remembrance and wreath laying, with "lights out" in City Hall for a period during the vigil.