Northumberland Day takes place annually on the last Sunday in May.
Anguilla Day takes place annually on 30 May.
On 30 May 1967 the Anguillans began a revolution that led to independence from the union of St Kitts-Nevis-Anguilla.
Wiltshire Day takes place annually on 5 June.
It commemorates the first raising of the Wiltshire Flag at County Hall, Trowbridge (2007).
The Falkland Islands celebrate Liberation Day annually on 14 June.
The date commemorates the liberation of the islands from Argentine military occupation in 1982.
Sussex Day takes place annually on 16 June, the feast of St Richard of Chichester (1197–1253), the patron saint of Sussex.
Also known as Richard de Wych, the saint was bishop of Chichester.
The official birthday of HM The King is listed as a flag flying day by UK government guidance.
The Union Flag is specified.
https://www.gov.uk/guidance/designated-days-for-union-flag-flying
The official birthday of HM The King is also listed as a flag flying day by Scottish government guidance.
The Scottish Saltire is specified.
https://www.gov.scot/publications/flag-flying-on-government-buildings/
Autistic Pride Day is an official flag flying day only in Scotland, not in the rest of the UK.
https://www.gov.scot/publications/flag-flying-on-government-buildings/