Elizabeth, 95, became the queen of Britain and more than a dozen other realms including Canada, Australia and New Zealand on the death of her father King George VI on Feb. 6, 1952, while she was in Kenya on an international tour.
The news was broken to her by her husband Prince Philip, who died last year aged 99 after more than seven decades by her side.
Elizabeth will mark 'Accession Day' in private as is customary, not viewing it as something to celebrate. But there will be four days of national events to mark her Platinum Jubilee in June.
"While it is a moment for national celebration, it will be a day of mixed emotions for Her Majesty as the day also marks 70 years since the death of her beloved father George VI," Prime Minister Boris Johnson told parliament on Wednesday, thanking the monarch for "her tireless service".
Elizabeth has continued to carry out official duties well into her 90s, but has been little seen in public since she spent a night in hospital last October for an unspecified ailment and was then instructed by doctors to rest.
However, Buckingham Palace on Friday released footage ahead of Sunday's landmark, showing her viewing items from previous royal jubilees, such as a fan given to her great-great-grandmother Queen Victoria to mark her 50th year on the throne in 1887, signed by family, friends and politicians.
Ironically Elizabeth was not destined to be monarch at her birth, and only became queen because her uncle Edward VIII abdicated to be with American divorcee Wallis Simpson.
But in 2015, she overtook Victoria as Britain's longest-reigning sovereign in a line that traces its origin back to Norman King William I and his 1066 conquest of England.
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