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The trees are blooming; the birds are singing; the newspaper society sections are thick with marriage announcements. As the last soggy weeks of spring give way to the balmy days of summer, wedding season has arrived  and with it, an onslaught of bachelor parties. With an estimated 2.2 million weddings in the U.S. each year, providing for the groom's send-off is big business. Dozens of websites cater to the needs of the bachelor-party planner (typically, the groom's best man). I-Volution Inc., which owns two of the largest bachelor-party sites on the Web, says its websites get about 4 million visitors a year  35% of whom focus on the Las Vegas packages. Just witness the success of the hit film The Hangover, whose tale of a prenuptial Las Vegas jaunt gone horribly awry has topped the box office for two straight weeks, pulling down more than $105.4 million. (See the top 10 non-emergency 911 calls.)

The bachelor party, however, goes back much further than you'd expect. It's rooted in ancient history  as early as the 5th century B.C. It is believed that the ancient Spartans were the first to make a celebration out of the groom's last night as a single man. Spartan soldiers held a dinner in their friend's honor and made toasts on his behalf  with, one assumes, a Spartan sense of decorum. Since then, the events have generally grown more raucous. In 1896, a stag party thrown by Herbert Barnum Seeley  a grandson of P.T. Barnum  for his brother was raided by police after rumors circulated that a famous belly dancer would be performing nude. Before his wedding to Gloria Hatrick, Jimmy Stewart's infamous bash at the Beverly Hills hangout Chasen's included midgets popping out of a serving dish.

The fun can get out of hand, however: in recent years, bachelor-party high jinks have led to numerous Hollywood breakups. Paris Hilton accused beau Paris Lastis of cheating on her at his bachelor party  an alleged indiscretion that similarly doomed Mario Lopez and Ali Landry. Nick Lachey's reported dalliance with a porn star at a friend's party  while it was denied  sparked rumors about a rift with wife Jessica Simpson before their eventual split in 2005. And Peter Berg's dark 1998 film Very Bad Things should be required viewing for grooms-to-be about the importance of good behavior (although it's probably not for their fiancées).

The term bachelor  previously meaning a young knight or a student with a bachelor's degree  first appeared in reference to an unmarried man in Geoffrey Chaucer's Canterbury Tales in the 14th century. The term bachelor party didn't appear until 1922, however, when it was first used in the Scottish publication Chambers's Journal of Literature, Science and Arts to describe a "jolly old" party. The event is known by different names in different countries: the stag party in the U.K., Ireland and Canada; the buck's party in Australia; and, with typical panache, the enterrement de vie de garçon in France (translation: "the burial of the life as a boy").

In the past, a bachelor party could commonly involve a black-tie dinner hosted by the groom's father, with toasts to the groom and the bride. The more recent traditions of hazing, humiliation and debauchery  often consuming entire weekends and involving travel to an exotic destination such as Las Vegas or its nearest available facsimile  became a staple of bad '80s sex comedies. (The 1984 Tom Hanks vehicle Bachelor Party hit the genre's perfecta, featuring beer, drugs, strippers, an ill-fated donkey and MTV video vixen Tawny Kitaen.) (Watch TIME's video "Beer Pong Strikes Back.")

By the sexual revolution of the 1960s, women had launched their own version of the prewedding festivities: the bachelorette party. Prior to the late 19th century, women were limited to bridal showers, the main function of which was to acquire a dowry and gifts to prepare them for marriage. Bachelorette parties allowed women the opportunity to express their own sexual freedom with drinking games and (male) strippers. Other couples, uncomfortable with the expectations of debauchery, celebrate their last night together in combined stag and doe parties  an idea that's grown popular as more couples live together and marry later in life. Bachelor parties are now as diverse as the bachelors involved, ranging from Las Vegas trips (losing teeth, dignity and sometimes the groom, as in The Hangover) to a casual party with friends and/or the fiancée. First and foremost, the event is an important step in saying goodbye to one's single life and relieving prewedding jitters. There doesn't even have to be a party: some men now opt for "groom's showers," in which they acquire their own dowry of foosball tables and power tools.

Read a review of "The Hangover's Bro-Magnon Bromance."