"Hare Hare Yukai" (Japanese: ハレ晴レユカイ lit. Sunny Sunny Happiness) is the ending theme song for The Melancholy of Haruhi Suzumiya, the 2006 anime based on the light novel series of the same name. Due to the song's catchiness and the simple but infectious dance sequence during ending credits, both became an immediate internet phenomenon upon it's release, with it spawning many covers, remixes, and parodies.
"Hare Hare Yukai" was created as the ending theme song for the anime The Melancholy of Haruhi Suzumiya, which was produced by studio Kyoto Animation (KyoAni) and premiered in Japan on April 9th, 2006. The song was written by lyricist Aki Hana and was performed by the show's voice actresses Aya Hirano, Minori Chihara, and Yuko Goto. For the ending credits, KyoAni decided to take inspiration of dance choreographies from J-pop Idol groups and created the now memorable dance sequence featuring the show's main cast, who are known as the "S.O.S. Brigade."
When the show started airing, the song and dance quickly began gaining interest among anime otakus in Japan. Pretty soon, many of them would begin creating their own takes through animated parodies or live-action performances. Many of the covers were performed either by one person or with groups, often arranged via the internet through 2channel, and sharing them to YouTube.
A month after the anime's debut, the full version of the theme was officially released as a single in Japan on May 10th, 2006. It was an immediate hit. Thanks to a large fan campaign, the song reached to #5 on the Oricon weekly charts, selling over 34,000 copies in it's first week and a reported 123,000 sold in 2008. On July 10th, 2006, a series of music singles performed by the vocal cast were released, which featured covers of "Hare Hare Yukai" (shown below, right), with some featuring alternate lyrics to reflect their characters.
When the video-sharing site Nico Nico Douga was established in March 2007, Japanese users migrated to the newly formed site and began sharing covers and parodies of the song and dance. The song would end up playing a major part on Niconico with the animated parodies going on to popularized the trend of parodying the credit sequences of anime series, while the dance videos helped lay the foundation for the dance cover movement Odottemita (踊ってみた, I tried to dance). This has been recognized with it being included in the first series of Niconico medleys during mid 2007 and in many others as the years went on.
Through out the rest of 2007, the song was reaching it's peak of interest and recreations of the dance eventually began growing in large scales. These ranged from flash mob style performances in the streets of Akihabara all the way to a choreographed performance by the Cebu prison inmates in the Philippines.
Meanwhile, the song also attracted interest from outside of Japan. Around the same time of the premiere of the series, anime fans from English-speaking countries began creating their own parodies and dance covers, often time filmed in cosplay at conventions, uploading to sites like deviantART and YouTube.
Left: KyoAni All-Stars | Right: Mega Man
Left: Pokemon | Right: Hetalia
 Anime News Network – Haruhi CD Stays on Japanese Singles Chart for 100th Week / Posted on 07-17-2008