This bittersweet song was co-written by Australian singer-songwriter Sia Furler, whose credits include Rihanna's "Diamonds," David Guetta's "Titanium" and Katy Perry's "Chained to the Rhythm." The Aussie songstress has also experienced her own solo success with such hit tunes as "Chandelier" and "Cheap Thrills."

"Any association with Sia is a complete honor. We asked her for a song because we were such big fans of 'Titanium,'" Debbie Harry told Billboard magazine. "She was writing with Nick Valensi from the Strokes, they submitted a couple songs and [we picked] the one we liked, 'Best Day Ever.'"