The Queen will become the longest serving monarch in British history on Wednesday, overtaking the record previously held by her great-great-grandmother Queen Victoria.
Officials have said the event will be low-key, with a private family gathering but not a celebration.
On the day, the Queen will also officially open the £300m Borders Railway in Edinburgh.
What's happening on the day?
Events have been planned across the country and the Commonwealth to celebrate the landmark occasion.
River Thames Pageant
A flotilla, similar to that of the 2012 Diamond jubilee celebrations, will sail along the River Thames.
People are being encouraged to line the route from the starting point at Tower Bridge.
From midday historic vessels, leisure cruisers and passenger boats will sound their horns for one minute before sailing beneath the bridge on their way towards the Houses of Parliament.
In Edinburgh, a new exhibition charting the Queen's years on the throne will open its doors at Holyroodhouse.
Long To Reign Over Us, will feature a selection of photographs from 1952 to the present day.
Other local celebrations will include a bell ringing at Ripon Cathedral in North Yorkshire at the exact time the Queen surpasses the record.