The Birthday of the Prince of Wales 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
Under his Scottish title (Duke of Rothesay), the Birthday 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/
Mercia Day takes place annually on 22 June, the feast of St Alban.
St Alban is the first recorded British Christian martyr, by tradition beheaded in Verulamium (modern St Albans) during the third or fourth centuries.
The British Virgin Islands celebrate Territory Day annually on 1 July.
On this date in 1956 the islands became a separate British Overseas Territory.
The Isle of Man celebrates Tynwald Day (Manx: Laa Tinvaal) annually on 5 July, unless this date falls on a Saturday or Sunday.
In this case Tynwald Day takes place on the following Monday.
Black Country Day celebrates the installation of the first practical steam engine by Thomas Newcomen (1712).
Hampshire Day takes place annually on 15 July, the Feast Day of St Swithun.
St Swithun was Bishop of Winchester (852–63) and subsequently patron saint of Winchester Cathedral.
The Birthday of HM The Queen 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 Birthday of HM The Queen 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/
Historic County Flags Day takes place annually on 23 July, giving an opportunity for individuals and organisations to show pride in their historic county.
The date commemorates the creation of the Devon Flag (2002), triggering the movement to design a modern flag for each of the UK’s historic counties.
Since 2019 (not 2020) the flags have flown together in Parliament Square, London.
Buckinghamshire Day marks the origin of the Paralympic Games.
On the opening day of the London Olympics (1948), Stoke Mandeville Hospital hosted the first event ever organised for disabled athletes alongside an Olympic Games.
The Stoke Mandeville Games later became the Paralympic Games, which first took place in Rome (1960).
Yorkshire Day began in 1975 as a celebration launched by the Yorkshire Ridings Society – newly formed ‘a protest movement against the local government re-organisation of 1974’ – to promote the county of Yorkshire.
The King’s Own Yorkshire Light Infantry, now part of the The Rifles, already celebrated this date as Minden Day, marking their part in the battle of Minden (1759).
The six infantry regiments involved in that battle wear a rose in their headdress.
In the case of The Rifles, the rose is white.