Llywelyn was killed by English soldiers at Irfon Bridge near Cilmeri in Powys on 11 December, 1282 while opposing an invasion of Wales by Edward the 1st.
Blaenau Ffestiniog Town Council is due to raise the Welsh flag in remembrance of Gwenllian, the daughter of Llywelyn ap Gruffudd, the last leader of the native Welsh dynasties who held out against English conquest.
Gwenllian was born on 12 June, 1282, and after her father's death was imprisoned by the English in a convent near Sempringham in Lincolnshire until her death.
Llywelyn was killed by English soldiers at Irfon Bridge near Cilmeri in Powys on 11 December, 1282 while opposing an invasion of Wales by Edward the 1st.
She was only a few months old when her father was killed.
Her mother - Llywelyn's cousin, Eleanor de Montfort - had died while giving birth to her in the palace of Pen-y-Bryn, in Abergwyngregyn near Bangor, Gwynedd.
Llywelyn and Eleanor (the daughter of Simon de Montfort) were married in Worcester in 1278 following Eleanor's release at the end of a period of about three years as a prisoner of the English crown.
Gwenllian was the only child of the marriage and so there were no sons to inherit the title of Prince of Wales.
But as the daughter of Prince Llywelyn, Gwenllian was the heiress of the Princes of Gwynedd and the royal family of Aberffraw.
'Biological time-bomb'
As the Princess of Wales she represented considerable danger to the king of England. One modern commentator described her as "a biological time-bomb" who had to be hidden away in case she became a focus for Welsh independence.
On the orders of Edward I, she was sent to the Gilbertine priory at Sempringham in Lincolnshire.
Edward kept the title of 'Prince of Wales' for the crown, bestowing it upon his son Edward who was crowned in Caernarfon in 1301 aged 17 years. The tradition of calling the eldest son of the English king or queen the Prince of Wales has continued until the present day.
Gwenllian died at Sempringham in 1337, having been held prisoner for 54 years.
In recent years, several initiatives have been taken to commemorate Gwenllian. These have included erecting a memorial stone near Sempringham.
The commemoration organised by Blaenau Ffestiniog town council is the first by a local authority.