Dang, I listen to this song by Junko Yagami（八神純子）and I just can't believe that a high school student was able to create and sing this lovely piece of bossa nova. Yagami was only 16 years old when she wrote and composed "Ame no Hi no Hitorigoto" (Rainy Day Monologue) for release in December 1974. Yagami's debut as a singer was supposedly with her 1978 creation "Omoide wa Utsukushi Sugite" （思い出は美しすぎて）, so "Ame no Hi no Hitorigoto" has been seen as a pre-debut of sorts.
At the time, Yagami was attending Aichi Shukutoku High School in her native Nagoya but also started going to the Yamaha Vocal Talent School. In October 1974, she entered the 8th Annual Yamaha Popular Song Contest and won an Excellent Song prize with this particular song. A couple of months later, it was released as a single.
Despite the sunny bossa, the title and lyrics go into Yagami's love for rain. Perhaps compared to her "official" debut of "Omoide wa Utsukushi Sugite", her delivery may not have been as polished but the talent was no doubt there. I was also struck by the last line in the refrain where she says "If you are going to bring in the happiness, please bring it in drop by drop." That Latin sound would be one of her hallmarks for the first few years of her career.