The custom of Trooping the Colour dates back to the time of Charles II in the 17th. Century when the Colours of a regiment were used as a rallying point in battle and were therefore trooped in front of the soldiers every day to make sure that every man could recognise those of his own regiment. In London, the Foot Guards used to do this from 1755 onwards as part of their daily Guard Mounting on Horse Guards and the ceremonial of the modern Trooping the Colour parade is along similar lines. In 1805 the parade was for the first time carried out to celebrate the Sovereign's birthday.
This impressive display of pageantry is now held on the occasion of the Queen's Official Birthday. It takes place in June each year to celebrate the official Birthday of the Sovereign and is carried out by her personal troops, the Household Division, on Horse Guards Parade, with the Queen herself attending and taking the salute.
There are rehearsal events held on the two Saturdays before Trooping the Colour: The Major General's Review and The Colonel's Review.
Official Link: http://www.army.mod.uk/events/ceremonial/