The Trooping of the Colour is a military parade attended by the Queen on her official birthday. The name is due to the different colours of the regiments. The traditions dates back from Charles II in the 17th century when this parade was created in order to make sure that every single man would recognize his own regiment. After 1748, this parade would mark the King/Queen's official birthday.
Since 1987, The Queen has attended in a carriage rather than riding, which she did before that on 36 occasions, riding side-saddle and wearing the uniform of the regiment whose Colour was being trooped. The regiments take their turn for this honour in rotation as operational commitments permit.