The queen, now 90, rose to the throne in 1952 at the age of 25 upon the premature death of her father, King George VI.
As with previous milestones in her long reign, Queen Elizabeth is likely to spend Accession Day privately at her Sandringham Estate in Norfolk, eastern England.
On Sunday, however, she was seen smiling as well-wishers gave her flowers outside church.
Despite her reluctance to celebrate publicly, the occasion will be marked in Britain.
At the Tower of London, in the capital’s Green Park and all around the kingdom, gun salutes will be fired in tribute.
Sapphire is the 65th anniversary gemstone and a photograph has been reissued of the sovereign wearing a suite of sapphire jewellery given to her by her father as a wedding gift in 1947.
The 2014 portrait by David Bailey shows her wearing the glittering necklace, dating from 1850 and made of 16 large oblong sapphires surrounded by diamonds.
“She has very kind eyes with a mischievous glint. I’ve always liked strong women, and she is a very strong woman,” the photographer said at the time.