Queen Elizabeth II is celebrating her Sapphire Jubilee, marking 65 years on the throne on Monday, the longest-reigning British monarch in history.

The queen ascended to the throne after her father died on Feb. 6, 1952 when she was 25 years old.

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She had no scheduled events on the anniversary and instead spent the day at Sandringham, her country retreat in eastern England, CNN reported.

The queen has reached another “remarkable milestone,” British Prime Minster Theresa May said in a statement.

“I know the nation will join with me today in celebrating and giving thanks for the lifetime of service Her Majesty the Queen has given to our country and to the commonwealth.”