The monarch sends special messages to people celebrating notable birthdays and anniversaries. This has happened in England since 1917.
The King's messages consist of a card with a custom message. The card comes in a special envelope, which is sent through the normal postal channels.
Cards are sent to people on their 100th and 105th birthday and every year after. They're also sent to those on their diamond wedding (60th), 65th, 70th wedding anniversaries and every year after.
The delivery of these messages is arranged by the Anniversaries Office. This is part of the Private Secretary's Office in the Royal Household. It's based at Buckingham Palace.
The Pension Service informs them of birthdays for recipients of UK State pensions. There is no automatic alert from government records for wedding anniversaries. The reason for this is data protection.
Applying for a message
An application to Buckingham Palace should be made in advance of the day. This is to make sure that the message is sent in time.
You can find more details and apply for a birthday or anniversary message on the GOV.UK website.