In a September 2008 interview with Italy's Musica Metal, The Rasmus singer Lauri Ylonen stated that the idea for the song came to him nine years ago when Tarja Turunen was still the lead singer of Nightwish, but he only got around to recording the track during the sessions for last year's "Black Roses" CD. "I actually asked Tarja, but she said no!" Lauri revealed. "Then I called Anette, and she was like, 'Yeah, I would love to sing a song with you, I'm a big Rasmus fan, actually two years ago before I joined Nightwish, I was singing The Rasmus songs in karaoke bars!' so we went with her."

"October & April" was not included on "Black Roses" because the CD was a concept effort and the song didn't fit in with the overall idea for the album. Instead, the track will make its debut on The Rasmus' "Best of 2001-2009", a compilation of all the hit songs from their four latest albums, including the million-selling "In The Shadows", most of the singles and a few rarities.

Due on November 25, "Best of 2001-2009" features the following track listing:

01. In The Shadows
02. No Fear
03. Guilty (U.S. mix)
04. October & April (featuring Anette Olzon)
05. First Day Of My Life
06. Livin' In A World Without You
07. F-F-F-Falling
08. Chill
09. Immortal
10. Justify
11. Shot
12. Funeral Song
13. Ghost of Love
14. Open My Eyes (acoustic version)
15. In My Life
16. Ten Black Roses
17. Sail Away