The official birthday of the Sovereign(Queen or King) is marked each year by a military parade and march which is known as Trooping the Colour.

Every June a tradition takes place on Horse Guards Parade, Whitehall, in front of crowds of onlookers and is enjoyed by millions world-wide.  The troops participating in the parade are fully trained, operational troops from the Household Division.  Many of these troops have served in Iraq or Afghanistan.

The Queen's Colour of a battalion of Foot Guards is 'trooped' (carried along the ranks) each year before the Queen, or reigning Monarch.   Only one colour can be trooped at a time, and the five Household Regiments - Grenadier,  Coldstream, Scots, Irish and Welsh Guards take their turn each year.

The ceremony dates from two old military ceremonies, Trooping the Colour and Mounting The Queen's Guard.  The origin of the ceremony goes back to the early eighteenth century, and possibly even earlier, when the guards and sentries for the Royal Palaces and other important buildings in the capital were mounted daily on the parade ground by the Horse Guards building.
A feature of guard mounting was for the colours of the battalion to be carried (or 'trooped') slowly down the ranks so that they could be seen and recognised by the soldiers.  In 1748 it was ordered that this parade would also mark the official birthday of the Sovereign.   From the accession of George IV the parade became - with the few exceptions of two World Wars- an annual event.

Tickets for the seated stands at the Trooping The Colour are  available to the general public. 

Applications should be made by post in January and February only, enclosing a stamped, self-addressed envelope to:

    
The Brigade Major
    
Headquarters Household Division
    Horseguards
Whitehall
London
    SW1A 2AX
United Kingdom

    Applicants who are successful in the ballot will then be able to purchase tickets.

    If you are not sure what to write in your letter, here is an example:

    

[Your Name, address and contact information]

    

Dear Sirs
    I would like to apply for  ( insert the number of tickets) tickets for the seated stands at Trooping the Colour  in London in June 2009 (or year that applies).

    I have enclosed a stamped, self-addressed envelope and look forward to hearing back from you.  
  

    Yours Sincerely,
    Name printed, and Signature

If  you do not included the number of tickets, you will have to send the request again.  You will receive a standard letter (with a reference number) by return mail, giving you more details about the Queen's Birthday Parade, plus The Colonel's Review and The Major General's Review, and the Household Division Beating Retreat, which are similar events on earlier weekends in June.

If you are offered tickets you will receive an offer letter (probably in March) with payment details. Note, they can only accept checks UK pounds drawn on a British Bank.  Credit cards are not accepted.
  If you are lucky enough to get tickets, plan to arrive extra early.  You must wait in line, and pass through security before entry and this can take a long time.  You must apply in writing to buy tickets for the stands.  I just attended for the first time, and highly recommend attending.  The cost of a ticket is around £25.00.  Since the event is secured, it is difficult to see anything without a ticket.  You can see the parade down the Mall for free if you do not get a ticket. Plan to arrive very early to get a spot.