[3] In December, Shigeru Suzuki, who already had a reputation as a skilled guitarist in the band Skye, was invited to join after Hosono and Matsumoto heard him improvise over what would become "Juuni Gatsu no Ame no hi".

However, due to the band's nervousness and difficulties working with the original recording engineer Tamotsu Yoshida, sessions were temporarily halted.

[8] In December 1969, Matsumoto went to visit Ohtaki, who was staying at Blues Creation vocalist Fumio Nunoya's apartment in Wakabayashi, Setagaya.

Specifically, they take up the boredom of one who faces the New Year holiday alone, sitting by himself at his kotatsu after having abandoned his rural family home for a new life in the city.

[10] This album marked an important turning point in Japanese music history, as it sparked what would be known as the "Japanese-language rock controversy" (日本語ロック論争, Nihongo Rokku Ronsō).

[2][13] Tal Rosenberg of Pitchfork wrote that the album feels like the band was "trying to replicate Buffalo Springfield instead of taking inspiration from them.

In 2021, photographer Mike Nogami released the photobook Yudemen containing pictures he took of Happy End during the recording of the album and an interview with Shigeru Suzuki.

[1][15] The song "Juuni Gatsu no Ame no hi" appears diegetically in the Japan-only PlayStation 2 game Boku no Natsuyasumi 2, set in August 1975 in a fictional coastal town in southern Japan.