bachelor party

[Georgbund]

stag party held for a bachelor (usually on the night before he is married)

[MiguelANymn]

A party held for a man who is about to be married.

[Becky]

A bachelor party (in the United States and sometimes in Canada), also known as a stag weekend, stag do or stag party (in the United Kingdom, Commonwealth countries, and Ireland), or a bucks night (in Australia), is a party held/arranged by the man who is shortly to enter marriage.
A stag night is usually planned by the grooms friend or brother, occasionally with the assistance of a bachelor party planning company.
The first references to Western stag nights in the Oxford English Dictionary date to the 19th century. Traditionally, stag nights involved a black tie banquet hosted by the father of the groom that included a toast in honour of the groom and bride. Since the 1980s, some bachelor parties in the United States have involved vacationing to a foreign destination, or have featured female company such as strippers or topless waitresses.