http://www.royalparks.gov.uk/tourists/gun_salutes.cfm
Royal Gun Salutes mark special royal occasions. On these days salutes are fired from locations in London and other authorised stations in the United Kingdom and the Union flag is hoisted on government buildings.
In London, salutes are fired in Hyde Park and at The Tower of London however, on State Visits, at the State Opening of Parliament and for The Queen's Birthday Parade, Green Park is used instead of Hyde Park.
The number of rounds fired in a Royal Gun Salute depends on the place and occasion. The basic salute is 21 rounds. In Hyde Park and Green Park an extra 20 rounds are added because they are a Royal Park.
Royal Gun Salutes in Hyde Park:
Salutes are fired by the King's Troop Royal Horse Artillery (41 Gun Royal Salute) in Hyde Park at 12.00 noon. They occur on the following Royal anniversaries, however gun salutes are not fired on Sundays, so if the date falls on a Sunday, the salute will take place the next day.
6 February - Accession Day, celebrating Queen Elizabeth II's accession to the throne
21 April - Birthday of Her Majesty the Queen
2 June - Coronation Day , celebrating Queen Elizabeth II's coronation
http://www.landrovercentre.com/Interesting_vehicles/guntroop/Royal_Salutes.htm
In 1924, when the regular detachment from the Royal Artillery stationed at HM Tower of London was disbanded as part of government economies, the HAC took over the task of firing the salutes from this Royal Fortress. The Regiment had fired salutes from there on previous occasions, the first recorded being on 18 March 1771.
Whilst a Royal Salute normally comprises 21 guns, increased to 41 if fired from a Royal Residence, 62 guns are fired from the Tower on Royal anniversaries and 41 on Royal or State occasions. Early engravings show guns on both the battlement s of the Tower and an the wharf, and an early regulation stated that 21 rounds would be fired from the carronades and 41 discharges from the chamber guns. It is believed that the guns on the wharf were the carronades, and it has been suggested, though not verified, that they were fired to indicate the loyalty of the citizens of London to their Sovereign.
62-gun salutes are fired on the following anniversaries (or day following, if the anniversary falls on a Sunday) at 1300 hrs, 1 round every 10 seconds: Anniversary of Accession, Coronation, Birthday (and official birthday) of Sovereign, Birthday of Sovereign's Consort, Sovereign's parent.
41-gun salutes are fired on the following occasions, 1 round every 10 seconds: State Opening of Parliament (at 1200 hrs), a State Visit ("timed"), Royal Birth.
