• 9 January Birthday of the Duchess of Cambridge
• 20 January Birthday of the Countess of Wessex
• 6 February Her Majesty's Accession
• 19 February Birthday of the Duke of York
• 1 March St David's Day (in Wales only, see note 1)
• 10 March Birthday of the Earl of Wessex
• 13 March Commonwealth Day (second Monday in March)
• 17 March St. Patrick's Day (in Northern Ireland only, see note 5)
• 21 April Birthday of Her Majesty the Queen
• 23 April St George's Day (in England only, see note 1)
• 9 May Europe Day (see note 4)
• 2 June Coronation Day
• 10 June Birthday of the Duke of Edinburgh
• 17 June Official celebration of Her Majesty's birthday
• 21 June Birthday of the Duke of Cambridge
• 17 July Birthday of the Duchess of Cornwall
• 15 August Birthday of the Princess Royal
• 12 November Remembrance Day (second Sunday in November, see note 2)
• 14 November Birthday of the Prince of Wales
• 20 November Her Majesty's Wedding Day
• 30 November St Andrew's Day (in Scotland only, see note 1)

Also:

• The day of the opening of a Session of the Houses of Parliament by Her Majesty (see note 3)
• The day of the prorogation of a Session of the Houses of Parliament by Her Majesty (see note 3)

Notes

1. Where a building has two or more flag poles the appropriate national flag may be flown in addition to the union flag but not in a superior position. UK government buildings within the wider Whitehall area may fly the national flags alongside the union flag on their appropriate saint days.
2. Flags should be flown at full mast all day.
3. Flags should be flown on this day even if Her Majesty does not perform the ceremony in person. Flags should only be flown in the Greater London area.
4. Where the European flag is flown on this day, the Union Flag should fly alongside the European flag and, on UK government buildings that only have one flagpole, the Union Flag should take precedence.
5. The Union Flag only should be flown.

*source:  College of Arms