Civic Sunday will be marked in the island’s capital in September.
It will be the first time St Mary’s Catholic Church has hosted the occasion since it was granted Cathedral status.
The honour was bestowed by Pope Francis after Douglas was officially named a city by the late Queen Elizabeth II as part of her Platinum Jubilee celebrations.
The Church of Saint Mary of the Isle is now a co-cathedral to the Metropolitan Cathedral of Christ the King, based 80 miles away in Liverpool city centre and will be the first Catholic co-cathedral in the British Isles.
Co-cathedrals are rare in the Catholic Church, but exist when two dioceses, each with its own cathedral, are merged or when a single diocese spans two distinct civil jurisdictions.
At the time of the , Monsignor John Devine said: 'It is not just an honour for Douglas, but it acknowledges the unique faith story of the Isle of Man.
'Christianity in the Isle of Man traces its roots to the time of St Patrick and St Maughold in the 5th century.
‘But there is limited appreciation of the unique history and traditions of the Manx Church in the rest of the archdiocese.
‘However, rather than separating them further, the granting of co-cathedral status to St Mary’s will raise consciousness in Liverpool to the riches of the Manx church.'
The announcement was followed by royal visit in March when Queen Camilla journeyed to the capital to officially confer city status on Douglas.
Douglas City Council’s first civic Sunday will be held on Sunday, September 22 and will be led by the Mayor, Councillor Natalie Byron-Teare JP.
Representatives of other local authorities, government departments, ex-service and charitable organisations, and members of the public are also being invited to attend the event.
Following a parade from City Hall in Ridgeway Street, led by Douglas Town Band, the Civic Service will begin at 11am at St Mary’s Church, conducted by the Mayor’s Chaplain, the head of the Catholic Church in the Island, Monsignor John Devine.
Members of the public wishing to attend are asked to be at the Church no later than 10.30am.
Mrs Byron-Teare said: ‘It is most fitting that the city of Douglas now has a cathedral and we are delighted to be holding our annual civic service at St Mary’s and extend an open invitation to members of the public.
‘This will be an inclusive community event for people of all faiths or none.’
The mayor has selected three local causes for her 2024-25 charity appeal: Naseem’s Manx Brain Tumour Charity, The Manx Housing Trust and Samaritans Isle of Man, and is hosting a variety of fundraisers during her term of office.