Japanese festival clothing is based on styles traditionally worn by chonin (townspeople during the Edo Period). The happi is basically a workman's everyday jacket. A traditional workman's coat or jacket is also called hanten. This kimono-style short, slip-over coat usually featured the employer's crest or name on the back, an early form of advertising. The colorful happi is also worn during festivals and other fun events. For example, Japanese baseball fans often wear a bright happi when going to the games.
Happi coats make wonderful after shower robes, summer robes to lounge around the home and for use as a beach robe or pool robe. Happi refers to a short length over coat or robe with straight sleeves and an open front, often fastened with ties. Happi coats originated as Japanese over coats traditionally worn by shop keepers. Today, happi coats are the most popular attire for festivals, parties and sushi restaurants.
Relevance to Urusei Yatsura
Another curious article of clothing that American audiences might not recognize, but are worn often in Urusei Yatsura. You'll see low to middle-class characters like Ataru or Ryuunosuke often wear a jacket-type hanten around whenever it's cold outside. Other more flashy types of happi (with Japanese characters written all over) are used by certain shop and restaurant owners or during festival-type events. They appear often during the series.