The salute will take place at noon on Saturday and will be fired from the saluting battery.
Anyone who would like to view the ceremony from inside the walls must arrive at the Castle before 11:40, as the bridge will be closed for a short period to allow the salute to take place.
Head of Heritage Services Helen Glencross said Guernsey Museums will be offering free entry to all of its sites throughout the day.
“The Queen’s birthday is always an important occasion for us as a Crown Dependancy,” she said.
“We are pleased to be able to mark it at the castle with a 21-gun salute.”
Castle Cornet and Fort Grey will be open 10:00 – 16:00, while the German Naval Signals HQ will be open from 12:30 – 16:30.
Guernsey Museum and Art Gallery will be open 10:00 – 17:00, where two temporary exhibitions – ‘Looking for Love’ by Alec Soth and ‘Between the Tides’ by Trudi Shannon – are currently being shown.