It will take place at 2pm on Monday, January 20.
This is the first of many events which will mark this milestone during 2014.
The date of the service is on the 20th anniversary of the opening of Joseph Weld Hospice and the day the first patients were admitted to the hospice.
Weldmar is also celebrating the 30th anniversary of the establishment of the community specialist palliative care nursing service in 1984.
The Bishop of Salisbury will be giving the address and the Lord Lieutenant of Dorset, Mrs Valerie Pitt Rivers, will also be attending the service.
Following the service the Bishop of Salisbury will be making a private visit to the hospice to meet patients and staff.
Weldmar Hospicecare Trust is the result of three well known and much respected Dorset charities merging together to become the only organisation to provide end of life, hospice and palliative care in North, South and West Dorset. Weldmar’s special care is provided to patients and their families in their homes, in day hospice and community services in Weymouth, Dorchester and Shaftesbury and in Joseph Weld Hospice by its skilled clinical and support staff, helped by trained volunteers. A spokesman for the charity said: “Weldmar is very grateful to all its supporters and volunteers who have helped fund and provide its care for the last three decades and looks forward to continued support from the communities it serves.”