Answer by taking a quote from the following article:

Nightwish is a symphonic metal band from Kitee, Finland. The band was formed in 1996 by lead songwriter and keyboardist Tuomas Holopainen, guitarist Emppu Vuorinen, and former lead singer Tarja Turunen. The band soon picked up drummer Jukka Nevalainen, and then bassist Sami Vanska after the release of their debut album, Angels Fall First (1997). In 2001, Vanska was replaced by Marco Hietala, who also took over the male vocalist role previously filled by Holopainen or guest singers.

A new album, Once, was released on June 7, 2004, along with its first single, "Nemo". The single topped the charts in Finland and Hungary, and reached the charts in six additional countries. "Nemo" remains the band's most successful single release to date.  Once utilizes a full orchestra in nine of the eleven songs on the album. Unlike Century Child, Nightwish decided to look for an orchestra outside of Finland this time, choosing the London Philharmonic Orchestra. It is also their second album to feature a full-length song in Finnish, "Kuolema tekee taiteilijan" (English: "Death Makes an Artist"). Once has sold triple platinum in Finland, platinum in Germany, and Gold in 6 other countries, it also reached No. 1 in the Greek, Norwegian and German album charts, and charted the Top 10 in France, Hungary and Sweden. The following singles were: "Wish I Had an Angel" (featured on the soundtrack of the film Alone in the Dark), "Kuolema Tekee Taiteilijan" (released only in Finland and Japan) and "The Siren". Besides the commercial success, Once was also well received by critics, with many positive reviewers drawing comparisons with Oceanborn.  The success of the album allowed them to perform the Once World Tour, taking them to play in many countries the band had never visited before. Nightwish performed at the opening ceremony of the 2005 World Championships in Athletics, held in Helsinki, highlighting the acclaim the band had gained. A "best of" album, Highest Hopes, was released in September 2005. The compilation also featured a live cover "High Hopes" (from the Pink Floyd album The Division Bell) (sample). Besides "High Hopes", a remake of "Sleeping Sun" (from Oceanborn) was included on the album and released as a single. A video for the remake was shot, featuring a medieval battle, and can be found on the German release of the single and as a separate DVD released by Spinefarm.  The four other members of Nightwish had decided that it was best to continue Nightwish without Turunen. After a concert in the Hartwall Areena (Helsinki) on October 21, 2005, recorded for the live DVD End of an Era (released June 2006), they expressed their decision through an open letter which was given to Turunen by Holopainen, and afterwards posted on the band's website. It was written by Holopainen and signed by all four band members. The main justification given in the letter for Turunen's dismissal was that the band felt that both her husband Marcelo Cabuli (an Argentine businessman) and commercial interests had changed her attitude towards the band.  Turunen responded to the incident twice, during press conferences in Finland and Germany, saying that her dismissal came entirely as a shock to her, given that she had not been notified before the letter was given to her. She felt that the personal attacks on her husband were unwarranted, and that playing the issue out in public was "senselessly cruel". She expressed these feelings through her own open letter, which was published on her personal website, and through various TV, magazine, and newspaper interviews.

Any interesting information?
Once utilizes a full orchestra in nine of the eleven songs on the album.