Once Upon a Time is the sixth extended play (EP) by South Korean girl group Lovelyz.

[2] They start releasing the individual teaser picture for the album in the next day through the group's official accounts on Twitter and Facebook.

[3][4] The short version teaser of the title track was uploaded in YouTube and Twitter on May 14.

[5] The title track "Beautiful Days" was revealed with the release of the album's tracklist on May 16.

[7] Billboard's Tamar Herman described the lead single "Beautiful Days (When We Were Us)" as a bright and airy synth-pop track with quirky retro synths.