Also known as: ハレ晴レユカイ, ハルヒダンス
Hare Hare Yukai is the ending theme song for the anime Suzumiya Haruhi No Yuuutsu. It features an intricately choreographed dancing sequence that has spawned many parodies.
On Danbooru, this tag refers to any image that references the ending animation.
The ending animation: https://youtu.be/x2iKC0C32-g