A costume party (American English) or a fancy dress party (British English), mainly in contemporary Western culture, is a type of party where guests dress up in a costume.

United States

Costume Parties are especially popular in the United States around Halloween, when teenagers and adults who may be considered too old for trick-or-treating attend a costume party instead. Halloween costume parties are occasionally featured in popular movies, such as Mean Girls.

Costume parties are also popular during the Carnival season, such as at Mardi Gras. Costume parties are also held for popular science fiction and fantasy events, movie openings and book releases. Website theonering.net held a The Lord of the Rings dress Oscar party that was attended by Peter Jackson. Star Wars parties were held to celebrate the opening of Episode I. Many bookstores have held Harry Potter themed parties to celebrate the releases of the series' later novels, and some movie theaters have had Potter-themed celebrations as the movie adaptations have been released.

Larger scale 'parties' are often related to organised societies or conventions

Fancy Dress parties in Britain

The origins of fancy dress parties in the UK can in some respects be traced to masked balls of the 18th Century period,
although Fancy Dress parties of the modern kind are a largely Victorian conception expanded upon by the Edwardians.

Notable amongst early events in the 20th Century was the Chelsea Arts ball. Such events were often elaborate affairs and for the most part confined to those with considerable means.

Amongst the general population, costume parties also occurred with increasing frequency from the late 1940's onward, although for the most part the costumes were simple affairs until the mid 70's. Prior to 'cheap' costume imports from the Far East (late 1990's) most costumes were either hired, or home constructed. Retail purchased costumes is in respect of the UK, a largely modern phenomenon (late 1990's onward) although 'accessory' items had been available for some time.
Coupled with the modern trend in costume parties, 'retro' fashion as a costume theme ( such as a 70's or even 80s fancy dress ) are also popular in the UK, the costumes to some extent parodying or pastiching the fashions of earlier decades. Amongst the most popular parodied costumes are: Audrey Hepburn (as Holly Golightly; Madonna in her classic stage outfits and more recently Lady Gaga.

Fancy dress parties are popular year round in the UK. Bridget Jones features the popular and classic theme "Tarts and Vicars". Fancy dress parties are also a favorite of the British Royal Family. Prince William celebrated his 21st birthday with an "Out of Africa" theme, Princess Beatrice chose an 1888 themed party for her 18th birthday, and Freddie and Gabriella Windsor celebrated a joint birthday party with a pre-French Revolution courtly theme. Prince Harry infamously wore an Afrika Corps uniform with a Nazi armband to a "Colonials and Natives" themed party in January 2005. There was a large international outcry after the Sunday tabloid News of the World published a photo of him in the costume.

Japan

In Japan, costume parties are closely related to cosplay.