The phrase "ある日どこかで" refers to a Japanese title of a film known in English as "Somewhere in Time." It is a 1980 American science fiction romance film directed by Jeannot Szwarc and starring Christopher Reeve and Jane Seymour. The plot involves a playwright named Richard who time travels back to 1912 to meet a woman he fell in love with from a photograph. The movie is notable for its emotional story about love transcending time and has gained a cult following since its release.
Hence, "ある日どこかで映画" translates roughly to "[ Somewhere in Time movie](javascript:void(0))" in English.
