Princes William and Harry in more frivolous times.
The Greeks (the ancient ones) started it, though not the bacchanalian bingeing part (that's Roman). The 50BC Greek stag night was a banquet of celebration to toast the bridegroom's good fortune. The bride was expected to be suitably blushing and got a smelly bath and an early night.
As for Prince William, it was all very proper. The official spokesman said it all: "I can confirm that Prince William's stag party happened this weekend. It was an entirely private event and we don't intend to make any further comment." So, as we guessed, definitely not in Prague, Dublin or Tripoli (now that would be a stag party venue, especially for the cousins in Hereford).
The modern royals have never been gutter-coughing traffic-cone cuddlers. Even Prince Andrew has toed some sort of line. Andrew held his stag in Lord Lascelles' house in Notting Hill. Everything went well until uniformed police raided the aristocratic joint. Well, not exactly the police. Instead, Knacker of the Yard's SWAT team turned out to be Sarah Ferguson and her chums. Not quite a stripper-gram, but quite giggly.
In 1947, Prince William's grandpa, Prince Philip, had a good old-fashioned naval party - noisy but with a late bottled Loyal Toast and Immortal Memory. It was organized at the Dorchester Hotel on Park Lane by Philip's closest naval friend Commander Mike Parker - a good natured seadog and rabble-rouser much disliked along the Privy Purse Corridor at Buckingham Palace. The media gathered for what were then, society pictures. Parker suggested there should be an official picture of the media. The hacks fell for it. Parker and Co. smashed their flashbulbs - no pictures of the historic event.
When Prince Charles married Diana, he had a modest and very private party in White's, the St James's club haunted mostly by Tories whose families either own the country, run the country, or both. No flashbulbs. Later of course, there was a party for his second marriage. We think it was no more than fun.
The stag party can do wonders (or horrors) for the morning after the night before.
The then George, Prince of Wales (later George IV) had taken much drink when, in 1795, he married his first cousin, Caroline of Brunswick. "The vilest wretch this world was ever cursed with," he claimed. Having secretly married his twice widowed Catholic and loving mistress, Maria Fitzherbert, and having an affair with France's Lady Jersey, George was never entirely disposed towards Caroline - he only married her because she was rich and he was not. So, George went to the altar drunk, to the banquet drunker, and to the bed-chamber in Brighton, with a very royal wobble on. He is said to have collapsed in front of the fire only waking to pay his respects sufficiently for their child, Charlotte to be conceived.
The champion of stag nights must, of course, be Henry VIII. They were however, in the Greek style. Banquets - sometimes lasting two weeks. By number six, there was a set menu of song, drink, and groaning tables.
By the way, why "Stag" Night? It comes from Chaucer's English - the adult male and commonly, a castrated one. Not an easy thought on the night before.