Original: Lennon was inspired to write the song by a 19th century circus poster for Pablo Fanque's circus he purchased in an antique shop on 31 January 1967, while filming the promotional video for "Strawberry Fields Forever" in Kent.
Simplified: Lennon wrote the song taking inspiration from a nineteenth century circus poster for Pablo Fanque's circus which he purchased in an antique shop on 31 January 1967 while filming the promotional video for the song "Strawberry Fields Forever" in Kent.