The battle of Albuera (1811) took place during the Peninsular War, when Anglo-Portuguese forces defeated the French.
The 57th (West Middlesex) Foot suffered very heavy casualties during the battle and later celebrated the anniversary as a regimental day.
St Aldhelm (639–709) was Abbot of Malmesbury and Bishop of Sherborne.
St Augustine of Canterbury (d.604) became the first Archbishop of Canterbury in 597.
The Church of England considers him to be one of its founding fathers.
Wiltshire Day takes place annually on 5 June.
It commemorates the first raising of the Wiltshire Flag at County Hall, Trowbridge (2007).
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 birthday of the Duke of Rothesay (Prince of Wales) is 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/
The Birthday of the Prince of Wales is also 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
Shetland Day takes place annually on 21 June.
Shetland Islands Council chose this date simply because it it the longest day of the year.