Halcyon Days is the reissue of English singer and songwriter Ellie Goulding's second studio album, Halcyon (2012). It was released on 23 August 2013 by Polydor Records. Following the release of Halcyon, Goulding announced that she had plans to repackage the original album, with new songs that focused on electronic music. For the reissue Goulding recorded ten new songs and worked with a variety of producers, including Ryan Tedder, Calvin Harris and Greg Kurstin, to create her desired sound. Goulding and the producers created a synthpop album that takes influence from indie pop, dream pop, alternative dance and features positive lyrics in contrast to her previous releases.

Upon release, Halcyon Days was met with rave reviews from most music critics, who praised the album's musical expansion, its production and happier lyrical content. Commercially the reissue was a success and helped propel the original album on numerous charts. The lead single from the reissued album, "Burn", was released on 5 July 2013, becoming Goulding's first number-one single on the UK Singles Chart. The song also reached the top five in Germany and Ireland, and the top ten in Australia, France and New Zealand. "How Long Will I Love You" and "Goodness Gracious" were subsequently released as the second and third singles from Halcyon Days, respectively.