"Gunjō Biyori" (群青日和, "Ultramarine Weather"), also known by its English name "Ideal Days for Ultramarine", is the debut single by Japanese rock band Tokyo Jihen, led by musician Ringo Sheena. It was released on September 8, 2004, ten months after Sheena's solo single "Ringo no Uta".

Tokyo Jihen was first formed in 2003, after Ringo Sheena decided on members for her backing band for her Sugoroku Ecstasy tour. It had been Sheena's intention to be release music in a band since before she debuted in 1998. The band's name was publicised during the tour, however previous tours had featured backing bands with names, such as Gyakutai Glykogen for Gekokujyo Xstasy and Hatsuiku Status.

The band was officially announced as Sheena's main musical unit on May 31, 2004, and first performed at a series of summer music festivals: Meet the World Beat 2004 on July 25, 2004 at the Expo Commemoration Park in Osaka, July 30, 2004 at the Fuji Rock Festival in Niigata, and September 3, 2004 at the Fukuoka Sunset Live festival.

All 17 of the songs for the Kyōiku era were recorded over a period of four days. The song features lyrics by Sheena set to a melody composed by then band pianist H Zetto M. It was one of three songs from Kyōiku to be written in this style, the other two being "Genjitsu ni Oite" and "Service". The single's B-sides include a cover of the 1937 standard "The Lady Is a Tramp", and "Kao", a duet between Sheena and then guitarist Mikio Hirama, who wrote the song's music. Sheena stated that it had been "a dream of sorts" to release music that collaborated with a variety of songwriters. The single's song lengths are all palindromic, much like Sheena's album Kalk Samen Kuri no Hana (2003). Pianist H Zetto M gave Sheena a collection of demos before recording sessions started, and "Gunjō Biyori" was the first song present. Sheena felt as if he had written the song specifically for Sheena, and everything she was experiencing. "Kao" was a song Hirama initially gave Sheena during the Sugoroku Tour, though she felt she could not sing it. Eventually, the idea of singing the song as a duet was created, meaning the song became feasible.

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