The birthday of the Duchess of Rothesay (Princess of Wales) 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/
The birthday of the Countess of Forfar (Duchess of Edinburgh) 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/
The Pitcairn Islands celebrate Bounty Day annually on 23 January.
Bounty Day commemorates the burning of HMS Bounty by the mutineers in 1790.
The start of LGBT+ History Month 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/
Shropshire Day falls on the feast day of St Milburga, abbess of Wenlock Priory , who died on 23 February 715.
The saint was revered for having the gift of healing and restoring sight.
The town of Much Wenlock thus became a place of pilgrimage, with visitors flocking to St Milburga’s Well seeking cures for their ailments.
St David’s Day is Wales’ national day.
The Senedd Cymru flies the Welsh national flag – Y Ddraig Goch (The Red Dragon) – all year round.
On St David’s Day it also flies the flag of St David (below).
https://senedd.wales/media/hl5dtuct/foi-574-final-redacted-response-saes.pdf
St David’s Day is also listed as a flag flying day (for Wales only) by UK government guidance.
The Union Flag is specified.
https://www.gov.uk/guidance/designated-days-for-union-flag-flying
St Piran, the patron saint of tin miners, is one of Cornwall’s patron saints.
St Felix (d.647/8), the first bishop of the East Angles, introduced Christianity to the kingdom.
The birthday of the Earl of Forfar (Duke of Edinburgh) 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/
Commonwealth Day is the second Monday in March.
The UK government states that the Union Flag should be flown.
https://www.gov.uk/guidance/designated-days-for-union-flag-flying
The Scottish government states that the Commonwealth flag should be flown.
https://www.gov.scot/publications/flag-flying-on-government-buildings/
The Senedd Cymru states that the Commonwealth flag should be flown.
https://senedd.wales/media/hl5dtuct/foi-574-final-redacted-response-saes.pdf