When your head of state has a birthday, you don"t throw a surprise party. You summon the Household Division, clear the Mall and get the Red Arrows to write a message in the sky. Trooping the Colour is a centuries-old military tradition, held each June to mark the official birthday of the reigning monarch—even if their actual birthday is in, say, November. The real reason? The weather"s better. No one wants to troop anything through puddles. There"s marching, music, horses, salutes—and a good chance of spotting a royal in a bearskin or bonnet.