The music video for the title track was released on June 11, 2019, along with "Breakthrough". The song was also released as a digital download on the same day.
- "Happy Happy" - 3:26
- "The Best Thing I Ever Did" (Japanese ver.) - 3:32
- "Happy Happy (collapsedone Remix)" - 3:32
- "Happy Happy (Instrumental)" - 3:27
- DVD (Type A only)
- "Happy Happy" music video
- "Happy Happy" music video making movie
- DVD (Type B only)
- "Happy Happy" music video (Lip sync ver.)
- Jacket shooting making movie
Two single release events with the TWICE members (except Mina) were organized:
- On July 29, 2019 at Makuhari Messe in Chiba,
- On July 31, 2019 at Intex Osaka convention center.
Both events offered the possibility to buy some exclusive "Happy Happy" goods. 21 exclusive objects were available.
The goods were also available for sale online at the Once Japan official shop, from August 1 to 18. An additional item was added for the occasion.
To celebrate the release of &TWICE -Repackage-, the remaining "Happy Happy" items were later sold at a selection of Tower Records store in Japan, from December 27, 2019 to January 13, 2020.
- The single's tracklist originally included the instrumental version of "The Best Thing I Ever Did", but on May 25, 2019, it was announced that it would be replaced by "Happy Happy (collapsedone Remix)".
- ↑ TWICE Japan: TWICE Mina announcement and apology (in Japanese)
- ↑ TWICE Japan: TWICE Japan 4th single "Happy Happy" release event official goods lineup (in Japanese)
- ↑ TWICE Japan: TWICE Japan 4th single "Happy Happy" release event official merchandise online shopping starts (in Japanese)
- ↑ TWICE Japan: TWICE Official Goods will be sold at Tower Records stores! (in Japanese)
- ↑ TWICE Japan: Notification of partial change in the content of "Happy Happy" and "Breakthrough" (in Japanese)