Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
LOONA (stylized as LOOΠΔ, also known as 'Girl of the Month', Korean: 이달의 소녀) is a South Korean pre-debut girl group project created by Blockberry Creative (Korean: 블록베리 크리에이티브), a subsidiary of Polaris Entertainment. Each month, beginning in October 2016, a new member is revealed and releases a single featuring one solo song, and one song including the previously revealed members, both will be accompanied by music videos. This will continue for a whole year, and at the end of the project the twelve girls will debut as one group.
History
Pre-debut project
On September 28, 2016, Blockberry Creative released a teaser for the first music video, Heejin's "ViViD". On October 2, South Korean company Blockberry Creative announced through Naver that they would be debuting their first girl-group through a 12-month long pre-debut project. Following this announcement, teasers were released for first member Heejin, as well as the teaser for an additional music video for the acoustic mix of "ViViD", set in Paris, France. On October 4, the music video for "ViViD" and it's accompanying acoustic mix were released.
On October 28, the second member, Hyunjin, was revealed. On November 10, Hyunjin's debut single "Around You" was released in a 10-minute short film format music video. Six days later, the music video for Heejin and Hyunjin's collaboration song "I'll Be There", filmed in Tokyo, Japan, was released, along with the official music video for "Around You".
On December 3, the third member, Haseul, was announced. On December 11, the music video teaser for "Let Me In" was released, revealing Haseul's music video to have been filmed in Iceland. On December 14, the music video teaser for Heejin, Hyunjin, and Haseul's collaboration Christmas song "The Carol" was released, and the music video was revealed to have been filmed in London, England. On December 15, the music videos for "Let Me In" and "The Carol" were released.