The Queen will reach a new milestone when she overtakes Queen Victoria to become the oldest British monarch.
Her great-great grandmother, who was born on 24 May 1819, lived for 81 years, seven months and 29 days.
Buckingham Palace says the Queen will beat the record at about 1700 GMT, taking into account the times of their births and Victoria's death.
But the day will be business as usual for the Queen, as there will be no special event to mark the occasion.
The monarch, who was born on 21 April 1926, will spend the day on her normal duties and has no public engagements or audiences.