At least three guardsmen fainted during the military parade known as the Colonel's Review, in which more than 1,400 soldiers of the Household Division and the King's Troop Royal Horse Artillery were reviewed by the heir to the throne, who is honorary Colonel of the Welsh Guards. The event was a rehearsal for Trooping the Colour, an annual military parade held each June to mark the monarch’s official birthday.

from Times of India https://timesofindia.indiatimes.com/world/uk/troops-feel-the-heat-and-several-faint-as-prince-william-reviews-military-parade/articleshow/100901797.cms