Time Out says
The ceremony to celebrate the sovereign's official birthday is probably the most spectacular of London's annual traditions. Carried out by fully operational troops from the Household Division (Foot Guards and Household Cavalry), Trooping the Colour is a military tradition that dates back to the early eighteenth century. Applications for tickets for the seated stands take place in January and February. Applications should be made to The Brigade Major, Headquarters Household Division, Horse Guards, Whitehall, London SW1A 2AX, stating how many tickets are required (max 3 per application) and including an SAE. The ballot takes place in early March. Successful applicants will be notified and payment will then be requested. Spectators can also watch along the route down the Mall and at the end of Green Park (arrive early if you want to be far enough forward to get a decent view). A fly-past takes place at 1pm.