Guernsey Museums will be allowing free entry for visitors as part of plans to mark King Charles III's 2025 official birthday.
On 14 June, Castle Cornet will mark the birthday with the annual 21-gun salute performed by a volunteer gun crew at 12:00 BST.
Guernsey Museum at Candie and Castle Cornet would both be open from 10:00 BST to 17:00 to visitors, managers said.
Fort Grey would be open from 10:30 to 16:30, while the German Naval Signals HQ would be open from 10:30 to 15:00, they added.
Head of heritage services Helen Glencross said: "We encourage locals and visitors to take advantage of free entry this Saturday, and come and learn more about our museums."
The King has two birthdays.
He was actually born on 14 November 1948, but his official royal celebration is Trooping the Colour, which falls on the second Saturday of June.
It has been the official birthday of the reigning British monarch for more than 260 years.
