Kiku Day (PhD, London; MFA, Mills; BA, London) is a shakuhachi player and ethnomusicologist from Copenhagen Denmark, where she grew up with her Japanese and American parents. She is a Visiting Research Fellow at Goldsmiths, University of London

Day plays jinashi shakuhachi - an older type of shakuhachi, only consisting of the raw untreated bamboo in the bore as a contrast to the modern shakuhachi with a rebuild bore. She also plays bone and plant flutes, pitched stones.

She gave up her studies in classical Western flute to study honkyoku (classic solo repertoire of the komusō monks of the Fuke sect of Zen Buddhism) with the master Okuda Atsuya in Tokyo for 11 years. She represents Okuda's Zensabō style in Europe.

Day has since her return to Europe dedicated her life to the potential use of jinashi shakuhachi today. She is dedicated to creating a contemporary repertoire for this archaic shakuhachi. Several composers from different parts of the world have written for her, among others: Roxanna Panufnik, Takahashi Yūji, Frank Denyer, Vytautas Germanavicius, Mogens Christensen, Marisol Jimenez and Yumi Hara. 

Day has performed with performers such as Fred Frith, Joanna MacGregor, Mats Gustafsson and Joëlle Léandre, and as a soloist with Odense Symphony Orchestra playing Takemitsu Toru's November Steps and with choirs such as the Nonsuch Choir, Voces8 and Coro Casa da Musica at venues such as Southbank, UK; Disney Hall, USA and the Danish Radio Concert Hall. In January 2024, Day has performances in Royal Festival Hall and Carnegie Hall. 

Day is known for her innovative playing techniques in cross-cultural settings, especially in her collaborations with performers such as Hyelim Kim from South Korea and in Sonas Mutua – a duet with Marisol Jimenez from Mexico and a duet with Ayako Kataoka. She has furthermore explored soundscapes of antiquity with music anthropologist Eva Fock. She brings the instruments such as bone flutes and pitched stones into her innovative collaborations.

Day is a founding member and served 10 years as chairperson for the European Shakuhachi Society and served as the chair of the Executive Committee for the 7th World Shakuhachi Festival in London. She was the chair of the Executive Committee for the 7th World Shakuhachi Festival held at Goldsmiths, University of London and the one-day academic conference Shakuhachi Symposium held at SOAS, University of London.

Day has also published several articles on the shakuhachi.